Semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor layer over a substrate; a gate insulating film covering the semiconductor layer; a gate wiring including a gate electrode, which is provided over the gate insulating film and is formed by stacking a first conductive layer and a second conductive layer; an insulating film covering the semiconductor layer and the gate wiring including the gate electrode; and a source wiring including a source electrode, which is provided over the insulating film, is electrically connected to the semiconductor layer, and is formed by stacking a third conductive layer and a fourth conductive layer. The gate electrode is formed using the first conductive layer. The gate wiring is formed using the first conductive layer and the second conductive layer. The source electrode is formed using the third conductive layer. The source wiring is formed using the third conductive layer and the fourth conductive layer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor devices, display devices,light-emitting devices, and manufacturing methods thereof. Inparticular, the present invention relates to semiconductor devicesincluding circuits having thin film transistors (hereinafter referred toas TFTs) in which oxide semiconductor films are used for channelformation regions, and manufacturing methods thereof. For example, thepresent invention relates to electronic devices on which electro-opticdevices typified by liquid crystal display panels or light-emittingdisplay devices including organic light-emitting elements are mounted ascomponents.

2. Description of the Related Art

Thin film transistors (TFTs) in which silicon layers formed usingamorphous silicon or the like are used as channel layers have beenwidely used as switching elements in display devices typified by liquidcrystal display devices. Although thin film transistors formed usingamorphous silicon have low field effect mobility, the thin filmtransistors have an advantage that larger glass substrates can be used.

Further, in recent years, attention has been drawn to a technique bywhich a thin film transistor is formed using a metal oxide havingsemiconductor properties and such a transistor is used in an electronicdevice or an optical device. For example, it has been known that somemetal oxides such as tungsten oxide, tin oxide, indium oxide, and zincoxide have semiconductor properties. A thin film transistor in which atransparent semiconductor layer formed using such a metal oxide is usedfor a channel formation region has been disclosed (Reference 1).

In addition, a technique for increasing aperture ratio by formation of achannel layer of a transistor with the use of a light-transmitting oxidesemiconductor layer and formation of a gate electrode, a sourceelectrode, and a drain electrode with the use of light-transmittingtransparent conductive films has been studied (Reference 2).

When the aperture ratio is increased, light use efficiency is improved,so that power consumption and the size of display devices can bereduced. On the other hand, from the viewpoint of an increase in thesize of display devices and application to mobile devices, morereduction in power consumption and an increase in aperture ratio aredemanded.

Note that as a method for providing a metal auxiliary wiring for atransparent electrode of an electro-optical element, a method by whichthe metal auxiliary wiring is provided so as to overlap with an uppersurface of the transparent electrode or a lower surface of thetransparent electrode and to be electrically connected to thetransparent electrode has been known (for example, see Reference 3).

Note that a structure in which an additional capacitor electrodeprovided for an active matrix substrate is formed using a transparentconductive film of ITO, SnO₂, or the like and an auxiliary wiring formedusing a metal film is provided in contact with the additional capacitorelectrode in order to lower the electric resistance of the additionalcapacitor electrode has been known (for example, see Reference 4).

Note that it has been known that, as each of a gate electrode, a sourceelectrode, and a drain electrode of a field effect transistor formedusing an amorphous oxide semiconductor film, a transparent electrode ofindium tin oxide (ITO), indium zinc oxide, ZnO, SnO₂, or the like, ametal electrode of Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like, or ametal electrode of an alloy containing any of the above elements can beused; and, by staking two or more of these layers, contact resistancemay be lowered or interface intensity may be improved (for example, seeReference 5).

Note that it has been known that, as a material for each of a sourceelectrode, a drain electrode, and a gate electrode of a transistorformed using an amorphous oxide semiconductor, and an auxiliarycapacitor electrode, a metal such as indium (In), aluminum (Al), gold(Au), or silver (Ag), or an oxide material such as indium oxide (In₂O₃),tin oxide (SnO₂), zinc oxide (ZnO), cadmium oxide (CdO), cadmium indiumoxide (CdIn₂O₄), cadmium tin oxide (Cd₂SnO₄), or zinc tin oxide(Zn₂SnO₄) can be used; and the same material or different materials maybe used for the gate electrode, the source electrode, and the drainelectrode (for example, see References 6 and 7).

REFERENCE

Reference 1: Japanese Published Patent Application No. 2004-103957

Reference 2: Japanese Published Patent Application No. 2007-081362

Reference 3: Japanese Published Patent Application No. 2-082221

Reference 4: Japanese Published Patent Application No. 2-310536

Reference 5: Japanese Published Patent Application No. 2008-243928

Reference 6: Japanese Published Patent Application No. 2007-109918

Reference 7: Japanese Published Patent Application No. 2007-115807

SUMMARY OF THE INVENTION

It is an object of one embodiment of the present invention to provide asemiconductor device with low power consumption. Alternatively, it is anobject of one embodiment of the present invention to provide asemiconductor device having low wiring resistance. Alternatively, it isan object of one embodiment of the present invention to provide asemiconductor device manufactured at low cost. Alternatively, it is anobject of one embodiment of the present invention to provide asemiconductor device having high transmittance. Alternatively, it is anobject of one embodiment of the present invention to provide ahigh-definition semiconductor device. Alternatively, it is an object ofone embodiment of the present invention to provide a semiconductordevice with high aperture ratio. Alternatively, it is an object of oneembodiment of the present invention to provide a semiconductor devicehaving high storage capacitance. Alternatively, it is an object of oneembodiment of the present invention to provide a semiconductor devicewith less light leakage. Alternatively, it is an object of oneembodiment of the present invention to provide a semiconductor devicewith low feedthrough voltage. Alternatively, it is an object of oneembodiment of the present invention to provide a semiconductor devicewhere a depletion layer is easily formed.

One embodiment of the present invention is a semiconductor device whichincludes a semiconductor layer provided over a substrate having aninsulating surface, a first wiring which is electrically connected tothe semiconductor layer and includes a first electrode, an insulatingfilm formed so as to cover the semiconductor layer and the firstelectrode, and a second wiring which is provided over the semiconductorlayer with the insulating film interposed therebetween and includes asecond electrode. The first electrode includes a first conductive layer.The first wiring includes the first conductive layer and a secondconductive layer. The second electrode includes a third conductivelayer. The second wiring includes the third conductive layer and afourth conductive layer.

One embodiment of the present invention is a semiconductor device whichincludes a semiconductor layer provided over a substrate having aninsulating surface, a first wiring which is connected to thesemiconductor layer and includes a first electrode, an insulating filmformed so as to cover the semiconductor layer and the first electrode, asecond wiring which is provided over the semiconductor layer with theinsulating film interposed therebetween and includes a second electrode,and a third wiring. The first electrode includes a first conductivelayer. The first wiring includes the first conductive layer and a secondconductive layer. The second electrode includes a third conductivelayer. The second wiring includes the third conductive layer and afourth conductive layer. The third wiring includes a fifth conductivelayer and a sixth conductive layer.

In the above, the first conductive layer and the third conductive layerpreferably have light-transmitting properties. In addition, the electricconductivity of each of the second conductive layer and the fourthconductive layer is preferably higher than the electric conductivity ofthe first conductive layer, the third conductive layer, or alight-transmitting conductive layer. Further, the second conductivelayer and the fourth conductive layer preferably have light-blockingproperties.

Further, in the above, the semiconductor layer is preferably an oxidesemiconductor layer which contains indium, gallium, or zinc.

Note that as an example of an oxide semiconductor which can be used inthis specification, there is an oxide semiconductor represented byInMO₃(ZnO)_(m) (m>0). Here, M is one or more metal elements selectedfrom gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), or cobalt(Co). For example, the case where Ga is selected as M includes not onlythe case where only Ga is used but also the case where Ga and the abovemetal element other than Ga, such as Ni or Fe, are selected. Further, inthe oxide semiconductor, in some cases, a transitional metal elementsuch as Fe or Ni or an oxide of the transitional metal is contained asan impurity element in addition to the metal element contained as M. Inthis specification, among the oxide semiconductors, an oxidesemiconductor containing at least gallium as M is referred to as anIn-Ga—Zn—O-based oxide semiconductor, and a thin film formed using thematerial is referred to as an In-Ga—Zn—O-based non-single-crystal filmin some cases.

As well as the above oxide semiconductors, any of the following oxidesemiconductors can be used as the oxide semiconductor: anIn—Sn—Zn—O-based oxide semiconductor; an In—Al—Zn—O-based oxidesemiconductor; a Sn-Ga—Zn—O-based oxide semiconductor; anAl-Ga—Zn—O-based oxide semiconductor; a Sn—Al—Zn—O-based oxidesemiconductor; an In—Zn—O-based oxide semiconductor; a Sn—Zn—O-basedoxide semiconductor; an Al—Zn—O-based oxide semiconductor; an In—O-basedoxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-basedoxide semiconductor. By addition of an impurity which suppressescrystallization to keep an amorphous state to these oxidesemiconductors, characteristics of thin film transistors can bestabilized.

Note that a semiconductor layer used in one embodiment of the presentinvention may have light-transmitting properties. For example, an oxidesemiconductor can be used for a light-transmitting semiconductor layer.Alternatively, as well as an oxide semiconductor, any of a crystallinesemiconductor (a single crystal semiconductor or a polycrystallinesemiconductor), an amorphous semiconductor, a microcrystallinesemiconductor, an organic semiconductor, and the like may be used.

Further, in the above, with the use of a multi-tone mask for processingthe first conductive layer, the second conductive layer, and the like, alight-transmitting region (a region with high transmittance) and alight-blocking region (a region with low transmittance) can be formedwith one mask (reticle). Accordingly, the light-transmitting region (theregion with high transmittance) and the light-blocking region (theregion with low transmittance) can be formed without an increase in thenumber of masks.

Note that in this specification, a semiconductor device refers to alldevices that can function by utilizing semiconductor properties, andsemiconductor circuits, display devices, electro-optic devices,light-emitting display devices, and electronic devices are allsemiconductor devices.

Note that in this specification, a display device refers to an imagedisplay device, a light-emitting device, or a light source (including alighting device). Further, a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP which is provided with a printed wiring board at an end thereof;and a module having an integrated circuit (IC) which is directly mountedon a display element by a chip on glass (COG) method are all displaydevices.

Note that a variety of switches can be used as a switch. For example, anelectrical switch, a mechanical switch, or the like can be used. Thatis, any element can be used as long as it can control a current flow,without limitation to a certain element. For example, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, an MIM (metal insulator metal)diode, an MIS (metal insulator semiconductor) diode, or adiode-connected transistor), or the like can be used as a switch.Alternatively, a logic circuit in which such elements are combined canbe used as a switch.

An example of a mechanical switch is a switch formed using a MEMS (microelectro mechanical system) technology, such as a digital micromirrordevice (DMD). Such a switch includes an electrode which can be movedmechanically, and operates by controlling conduction and non-conductionin accordance with movement of the electrode.

In the case of using a transistor as a switch, the polarity(conductivity type) of the transistor is not particularly limited to acertain type because it operates just as a switch. However, a transistorhaving polarity with smaller off-state current is preferably used whenthe amount of off-state current is to be suppressed. Examples of atransistor with smaller off-state current are a transistor provided withan LDD region, a transistor with a multi-gate structure, and the like.Further, an n-channel transistor is preferably used when a potential ofa source terminal of the transistor which is operated as a switch isclose to a potential of a low-potential-side power supply (e.g., Vss,GND, or 0 V). On the other hand, a p-channel transistor is preferablyused when the potential of the source terminal is close to a potentialof a high-potential-side power supply (e.g., Vdd). This is because theabsolute value of gate-source voltage can be increased when thepotential of the source terminal of the n-channel transistor is close toa potential of a low-potential-side power supply and when the potentialof the source terminal of the p-channel transistor is close to apotential of a high-potential-side power supply, so that the transistorcan be more accurately operated as a switch. This is also because thetransistor does not often perform source follower operation, so thatreduction in output voltage does not often occur.

Note that a CMOS switch may be used as a switch by using both ann-channel transistor and a p-channel transistor. By using a CMOS switch,the switch can be more accurately operated as a switch because currentcan flow when either the p-channel transistor or the n-channeltransistor is turned on. For example, voltage can be appropriatelyoutput regardless of whether voltage of an input signal to the switch ishigh or low. In addition, since the voltage amplitude value of a signalfor turning on or off the switch can be made smaller, power consumptioncan be reduced.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal for controlling conduction (a gate terminal).On the other hand, when a diode is used as a switch, the switch does notinclude a terminal for controlling conduction in some cases. Therefore,when a diode is used as a switch, the number of wirings for controllingterminals can be further reduced as compared to the case of using atransistor.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be interposed between elements having a connection relationillustrated in drawings and texts, without limitation to a predeterminedconnection relation, for example, the connection relation illustrated inthe drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection between A and B(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) may be connected between A and B. Alternatively, in thecase where A and B are functionally connected, one or more circuitswhich enable functional connection between A and B (e.g., a logiccircuit such as an inverter, a NAND circuit, or a NOR circuit; a signalconverter circuit such as a DA converter circuit, an AD convertercircuit, or a gamma correction circuit; a potential level convertercircuit such as a power supply circuit (e.g., a dc-dc converter, astep-up dc-dc converter, or a step-down dc-dc converter) or a levelshifter circuit for changing a potential level of a signal; a voltagesource; a current source; a switching circuit; an amplifier circuit suchas a circuit which can increase signal amplitude, the amount of current,or the like, an operational amplifier, a differential amplifier circuit,a source follower circuit, or a buffer circuit; a signal generationcircuit; a memory circuit; and/or a control circuit) may be connectedbetween A and B. For example, in the case where a signal output from Ais transmitted to B even when another circuit is interposed between Aand B, A and B are functionally connected.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuitinterposed therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected withanother circuit interposed therebetween), and the case where A and B aredirectly connected (i.e., the case where A and B are connected withoutanother element or another circuit interposed therebetween) are includedtherein. That is, when it is explicitly described that “A and B areelectrically connected”, the description is the same as the case whereit is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes and can include various elements. Forexample, a display medium, whose contrast, luminance, reflectivity,transmittance, or the like changes by electromagnetic action, such as anEL (electroluminescence) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor which emits light depending on theamount of current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a digital micromirror device (DMD), apiezoelectric ceramic display, or a carbon nanotube can be used as adisplay element, a display device, a light-emitting element, or alight-emitting device. Note that display devices having EL elementsinclude an EL display; display devices having electron emitters includea field emission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like; displaydevices having liquid crystal elements include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection liquid crystal display); displaydevices having electronic ink or electrophoretic elements includeelectronic paper.

Note that an EL element is an element including an anode, a cathode, andan EL layer interposed between the anode and the cathode. Note that asan EL layer, a layer utilizing light emission (fluorescence) from asinglet exciton, a layer utilizing light emission (phosphorescence) froma triplet exciton, a layer utilizing light emission (fluorescence) froma singlet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed using an organic material, a layer formed usingan inorganic material, a layer formed using an organic material and aninorganic material, a layer including a high-molecular material, a layerincluding a low-molecular material, a layer including a high-molecularmaterial and a low-molecular material, or the like can be used. Notethat the present invention is not limited to this, and a variety of ELelements can be used as an EL element.

Note that an electron emitter is an element in which electrons areextracted by high electric field concentration on a cathode. Forexample, as an electron emitter, a Spindt type, a carbon nanotube (CNT)type, a metal-insulator-metal (MIM) type in which a metal, an insulator,and a metal are stacked, a metal-insulator-semiconductor (MIS) type inwhich a metal, an insulator, and a semiconductor are stacked, a MOStype, a silicon type, a thin film diode type, a diamond type, a thinfilm type in which a metal, an insulator, a semiconductor, and a metalare stacked, a HEED type, an EL type, a porous silicon type, asurface-conduction (SCE) type, or the like can be used. Note that thepresent invention is not limited to this, and a variety of elements canbe used as an electron emitter.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof liquid crystals and includes a pair of electrodes and liquidcrystals. Note that the optical modulation action of liquid crystals iscontrolled by an electric field applied to the liquid crystals(including a horizontal electric field, a vertical electric field, and adiagonal electric field). Note that the following can be used for aliquid crystal element: a nematic liquid crystal, a cholesteric liquidcrystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a plasma addressed liquidcrystal (PALC), a banana-shaped liquid crystal, and the like. Inaddition, the following can be used as a diving method of a liquidcrystal: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (opticallycompensated birefringence) mode, an ECB (electrically controlledbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, a blue phase mode, and thelike. Note that the present invention is not limited to this, and avariety of liquid crystal elements and driving methods thereof can beused as a liquid crystal element and a driving method thereof.

Note that electronic paper corresponds to a device for displaying imagesby molecules (a device which utilizes optical anisotropy, dye molecularorientation, or the like), a device for displaying images by particles(a device which utilizes electrophoresis, particle movement, particlerotation, phase change, or the like), a device for displaying images bymovement of one end of a film, a device for displaying images by usingcoloring properties or phase change of molecules, a device fordisplaying images by using optical absorption by molecules, or a devicefor displaying images by using self-light emission by combination ofelectrons and holes. For example, the following can be used for adisplay method of electronic paper: microcapsule electrophoresis,horizontal electrophoresis, vertical electrophoresis, a sphericaltwisting ball, a magnetic twisting ball, a columnar twisting ball, acharged toner, an electron powder and granular material, magneticelectrophoresis, a magnetic thermosensitive type, electro wetting,light-scattering (transparent-opaque change), a cholesteric liquidcrystal and a photoconductive layer, a cholesteric liquid crystaldevice, a bistable nematic liquid crystal, a ferroelectric liquidcrystal, a liquid crystal dispersed type with a dichroic dye, a movablefilm, coloring and decoloring properties of a leuco dye, photochromism,electrochromism, electrodeposition, flexible organic EL, and the like.Note that the present invention is not limited to this, and a variety ofelectronic paper and display methods thereof can be used as electronicpaper and a driving method thereof. Here, by using microcapsuleelectrophoresis, defects of electrophoresis, which are aggregation andprecipitation of phoresis particles, can be solved. Electron powder andgranular material has advantages such as high-speed response, highreflectivity, wide viewing angle, low power consumption, and memoryproperties.

Note that a plasma display panel has a structure where a substratehaving a surface provided with an electrode faces with a substratehaving a surface provided with an electrode and a minute groove in whicha phosphor layer is formed at a narrow interval and a rare gas is sealedtherein. Alternatively, the plasma display panel can have a structurewhere a plasma tube is sandwiched between film-form electrodes from thetop and the bottom. The plasma tube is formed by sealing a dischargegas, RGB fluorescent materials, and the like inside a glass tube. Notethat the plasma display panel can perform display by application ofvoltage between the electrodes to generate an ultraviolet ray so that aphosphor emits light. Note that the plasma display panel may be aDC-type PDP or an AC-type PDP. Here, as a driving method of the plasmadisplay panel, AWS (address while sustain) driving, ADS (address displayseparated) driving in which a subframe is divided into a reset period,an address period, and a sustain period, CLEAR (high-contrast and lowenergy address and reduction of false contour sequence) driving, ALIS(alternate lighting of surfaces) method, TERES (technology of reciprocalsustainer) driving, or the like can be used. Note that the presentinvention is not limited to this, and a variety of driving methods canbe used as a driving method of a plasma display panel.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source of a display device in which alight source is needed, such as a liquid crystal display (e.g., atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display), a displaydevice including a grating light valve (GLV), or a display deviceincluding a digital micromirror device (DMD). Note that the presentinvention is not limited to this, and a variety of light sources can beused as a light source.

Note that a variety of transistors can be used as a transistor, withoutlimitation to a certain type. For example, a thin film transistor (TFT)including a single crystal semiconductor film or a non-single-crystalsemiconductor film typified by an amorphous silicon film, apolycrystalline silicon film, or a microcrystalline (also referred to asmicrocrystal, nanocrystal, or semi-amorphous) silicon film, or the likecan be used. In the case of using the TFT, there are various advantages.For example, since the TFT can be formed at temperature which is lowerthan that of the case of using single crystal silicon, manufacturingcost can be reduced or a manufacturing apparatus can be made larger.Since the manufacturing apparatus can be made larger, the TFT can beformed using a large substrate. Therefore, many display devices can beformed at the same time at low cost. In addition, since themanufacturing temperature is low, a substrate having low heat resistancecan be used. Therefore, the transistor can be formed using alight-transmitting substrate. Further, transmission of light in adisplay element can be controlled by using the transistor formed usingthe light-transmitting substrate. Alternatively, part of a film includedin the transistor can transmit light because the thickness of thetransistor is small. Therefore, the aperture ratio can be improved.

Note that by using a catalyst (e.g., nickel) in the case of formingpolycrystalline silicon, crystallinity can be further improved and atransistor having excellent electrical characteristics can be formed.Accordingly, a gate driver circuit (e.g., a scan line driver circuit), asource driver circuit (e.g., a signal line driver circuit), and/or asignal processing circuit (e.g., a signal generation circuit, a gammacorrection circuit, or a DA converter circuit) can be formed using thesame substrate as a pixel portion.

Note that by using a catalyst (e.g., nickel) in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electrical characteristics can be formed. Inthis case, crystallinity can be improved by just performing heattreatment without performing laser irradiation. Accordingly, a gatedriver circuit (e.g., a scan line driver circuit) and part of a sourcedriver circuit (e.g., an analog switch) can be formed using the samesubstrate as a pixel portion. In addition, in the case of not performinglaser irradiation for crystallization, unevenness in crystallinity ofsilicon can be suppressed. Therefore, high-quality images can bedisplayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without use of a catalyst (e.g., nickel).

Note that it is preferable that crystallinity of silicon be improved topolycrystal, microcrystal, or the like in the whole panel; however, thepresent invention is not limited to this. Crystallinity of silicon maybe improved only in part of the panel. Selective improvement incrystallinity is possible by selective laser irradiation or the like.For example, only a peripheral driver circuit region excluding pixelsmay be irradiated with laser light. Alternatively, only a region of agate driver circuit, a source driver circuit, or the like may beirradiated with laser light. Alternatively, only part of a source drivercircuit (e.g., an analog switch) may be irradiated with laser light.Accordingly, crystallinity of silicon can be improved only in a regionin which a circuit needs to be operated at high speed. Since a pixelregion is not particularly needed to be operated at high speed, even ifcrystallinity is not improved, the pixel circuit can be operated withoutproblems. Since a region whose crystallinity is improved is small,manufacturing steps can be decreased, throughput can be increased, andmanufacturing cost can be reduced. Since the number of necessarymanufacturing apparatus is small, manufacturing cost can be reduced.

A transistor can be formed using a semiconductor substrate, an SOIsubstrate, or the like. Thus, a transistor with fewer variations incharacteristics, sizes, shapes, or the like, with high current supplycapability, and with a small size can be formed. By using such atransistor, power consumption of a circuit can be reduced or a circuitcan be highly integrated.

A transistor including a compound semiconductor or an oxidesemiconductor, such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO, SnO, TiO,or AlZnSnO (AZTO), a thin film transistor obtained by thinning such acompound semiconductor or an oxide semiconductor, or the like can beused. Thus, manufacturing temperature can be lowered and for example,such a transistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used notonly for a channel formation region of the transistor but also for otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used for a resistor, a pixel electrode, or alight-transmitting electrode. Further, since such an element can beformed at the same time as the transistor, cost can be reduced.

A transistor or the like formed by an inkjet method or a printing methodcan be used. Thus, a transistor can be formed at room temperature, canbe formed at a low vacuum, or can be formed using a large substrate.Since the transistor can be formed without use of a mask (reticle), thelayout of the transistor can be easily changed. Further, since it is notnecessary to use a resist, material cost is reduced and the number ofsteps can be reduced. Furthermore, since a film is formed only in anecessary portion, a material is not wasted as compared to amanufacturing method by which etching is performed after the film isformed over the entire surface, so that cost can be reduced.

A transistor or the like including an organic semiconductor or a carbonnanotube can be used. Thus, such a transistor can be formed over aflexible substrate. A semiconductor device formed using such a substratecan resist shocks.

Further, transistors with a variety of structures can be used. Forexample, a MOS transistor, a junction transistor, a bipolar transistor,or the like can be used as a transistor. By using a MOS transistor, thesize of the transistor can be reduced. Thus, a plurality of transistorscan be mounted. By using a bipolar transistor, large current can flow.Thus, a circuit can be operated at high speed.

Note that a MOS transistor, a bipolar transistor, and the like may beformed over one substrate. Thus, reduction in power consumption,reduction in size, high-speed operation, and the like can be achieved.

Furthermore, a variety of transistors can be used.

Note that a transistor can be formed using a variety of substrates,without limitation to a certain type. As the substrate, a single crystalsubstrate (e.g., a silicon substrate), an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a metal substrate, astainless steel substrate, a substrate including stainless steel foil, atungsten substrate, a substrate including tungsten foil, a flexiblesubstrate, or the like can be used, for example. As a glass substrate, abarium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, or the like can be used, for example. For a flexiblesubstrate, a flexible synthetic resin such as plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES), or acrylic can be used, for example.Alternatively, an attachment film (formed using polypropylene,polyester, vinyl, polyvinyl fluoride, polyvinyl chloride, or the like),paper of a fibrous material, a base material film (formed usingpolyester, polyamide, polyimide, an inorganic vapor deposition film,paper, or the like), or the like can be used. Alternatively, thetransistor may be formed using one substrate, and then, the transistormay be transferred to another substrate. A single crystal substrate, anSOI substrate, a glass substrate, a quartz substrate, a plasticsubstrate, a paper substrate, a cellophane substrate, a stone substrate,a wood substrate, a cloth substrate (including a natural fiber (e.g.,silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like), a leather substrate, a rubbersubstrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate to whichthe transistor is transferred. Alternatively, a skin (e.g., epidermis orcorium) or hypodermal tissue of an animal such as a human being can beused as a substrate to which the transistor is transferred.Alternatively, the transistor may be formed using one substrate and thesubstrate may be thinned by polishing. A single crystal substrate, anSOI substrate, a glass substrate, a quartz substrate, a plasticsubstrate, a stainless steel substrate, a substrate including astainless steel foil, or the like can be used as a substrate to bepolished. By using such a substrate, a transistor with excellentproperties or a transistor with low power consumption can be formed, adevice with high durability and high heat resistance can be provided, orreduction in weight or thickness can be achieved.

Note that the structure of a transistor can be a variety of structures,without limitation to a certain structure. For example, a multi-gatestructure having two or more gate electrodes can be used. By using themulti-gate structure, a structure where a plurality of transistors areconnected in series is provided because channel regions are connected inseries. With the multi-gate structure, the amount of off-state currentcan be reduced and the withstand voltage of the transistor can beincreased (reliability can be improved). Further, with the multi-gatestructure, drain-source current does not fluctuate very much even whendrain-source voltage fluctuates when the transistor operates in asaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having an extremely large resistance value can be realized.Accordingly, a differential circuit or a current mirror circuit havingexcellent properties can be realized.

As another example, a structure where gate electrodes are formed aboveand below a channel can be used. By using the structure where gateelectrodes are formed above and below the channel, a channel region isincreased, so that the amount of current can be increased.Alternatively, by using the structure where gate electrodes are formedabove and below the channel, a depletion layer can be easily formed, sothat subthreshold swing can be improved. Note that when the gateelectrodes are formed above and below the channel, a structure where aplurality of transistors are connected in parallel is provided.

A structure where a gate electrode is formed above a channel region, astructure where a gate electrode is formed below a channel region, astaggered structure, an inverted staggered structure, a structure wherea channel region is divided into a plurality of regions, or a structurewhere channel regions are connected in parallel or in series can beused. Alternatively, a structure where a source electrode or a drainelectrode overlaps with a channel region (or part of it) can be used. Byusing the structure where the source electrode or the drain electrodeoverlaps with the channel region (or part of it), unstable operation dueto accumulation of electric charge in part of the channel region can beprevented. Alternatively, a structure where an LDD region is providedcan be used. By providing the LDD region, the amount of off-statecurrent can be reduced or the withstand voltage of the transistor can beincreased (reliability can be improved). Further, by providing the LDDregion, drain-source current does not fluctuate very much even whendrain-source voltage fluctuates when the transistor operates in thesaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained.

Note that a variety of transistors can be used as a transistor, and thetransistor can be formed using a variety of substrates. Accordingly, allthe circuits that are necessary to realize a predetermined function canbe formed using the same substrate. For example, all the circuits thatare necessary to realize the predetermined function can be formed usinga glass substrate, a plastic substrate, a single crystal substrate, anSOI substrate, or any other substrate. When all the circuits that arenecessary to realize the predetermined function are formed using thesame substrate, cost can be reduced by reduction in the number ofcomponents or reliability can be improved by reduction in the number ofconnections to circuit components. Alternatively, some of the circuitswhich are necessary to realize the predetermined function can be formedusing one substrate and some of the circuits which are necessary torealize the predetermined function can be formed using anothersubstrate. That is, not all the circuits that are necessary to realizethe predetermined function are required to be formed using the samesubstrate. For example, some of the circuits which are necessary torealize the predetermined function can be formed by transistors using aglass substrate and some of the circuits which are necessary to realizethe predetermined function can be formed using a single crystalsubstrate, so that an IC chip formed by a transistor using the singlecrystal substrate can be connected to the glass substrate by COG (chipon glass) and the IC chip may be provided over the glass substrate.Alternatively, the IC chip can be connected to the glass substrate byTAB (tape automated bonding) or a printed wiring board. When some of thecircuits are formed using the same substrate in this manner, cost can bereduced by reduction in the number of components or reliability can beimproved by reduction in the number of connections to circuitcomponents. Alternatively, when circuits with high driving voltage andhigh driving frequency, which consume large power, are formed using asingle crystal substrate instead of forming such circuits using the samesubstrate, and an IC chip formed by the circuits is used, for example,the increase in power consumption can be prevented.

Note that one pixel corresponds to one element whose brightness can becontrolled. Therefore, for example, one pixel corresponds to one colorelement and brightness is expressed with the one color element.Accordingly, in that case, in the case of a color display device havingcolor elements of R (red), G (green), and B (blue), the minimum unit ofan image is formed of three pixels of an R pixel, a G pixel, and a Bpixel. Note that the color elements are not limited to three colors, andcolor elements of more than three colors may be used or a color otherthan RGB may be used. For example, RGBW (W corresponds to white) can beused by adding white. Alternatively, one or more colors of yellow, cyan,magenta, emerald green, vermilion, and the like can be added to RGB.Alternatively, a color similar to at least one of R, G, and B can beadded to RGB. For example, R, G, B1, and B2 may be used. Although bothB1 and B2 are blue, they have slightly different frequencies. In asimilar manner, R1, R2, G, and B can be used. By using such colorelements, display which is closer to the real object can be performedand power consumption can be reduced. As another example, in the case ofcontrolling brightness of one color element by using a plurality ofregions, one region can correspond to one pixel. Therefore, for example,in the case of performing area ratio gray scale display or in the caseof including a subpixel, a plurality of regions which control brightnessare provided in each color element and gray levels are expressed withthe whole regions. In this case, one region which controls brightnesscan correspond to one pixel. Thus, in that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof regions which control brightness are provided in one color element,these regions may be collected and one color element may be referred toas one pixel. Thus, in that case, one color element includes one pixel.Alternatively, in the case where brightness is controlled in a pluralityof regions in each color element, the size of regions which contributeto display is varied depending on pixels in some cases. Alternatively,in the plurality of regions which control brightness in each colorelement, signals supplied to each of the plurality of regions may beslightly varied so that the viewing angle is widened. That is,potentials of pixel electrodes included in the plurality of regionsprovided in each color element can be different from each other.Accordingly, voltage applied to liquid crystal molecules are varieddepending on the pixel electrodes. Therefore, the viewing angle can bewidened.

Note that explicit description “one pixel (for three colors)”corresponds to the case where three pixels of R, G, and B are consideredas one pixel. Explicit description “one pixel (for one color)”corresponds to the case where the plurality of regions are provided ineach color element and collectively considered as one pixel.

Note that pixels are provided (arranged) in matrix in some cases. Here,description that pixels are provided (arranged) in matrix includes thecase where the pixels are arranged in a straight line and the case wherethe pixels are arranged in a jagged line, in a longitudinal direction ora lateral direction. Thus, for example, in the case of performing fullcolor display with three color elements (e.g., RGB), the following casesare included: the case where the pixels are arranged in stripes and thecase where dots of the three color elements are arranged in a deltapattern. In addition, the case is also included in which dots of thethree color elements are provided in Bayer arrangement. Note that thesize of display regions may be different between dots of color elements.Thus, power consumption can be reduced or the life of a display elementcan be prolonged.

Note that an active matrix method in which an active element is includedin a pixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In an active matrix method, as an active element (a non-linear element),not only a transistor but also a variety of active elements (non-linearelements) can be used. For example, an MIM (metal insulator metal), aTFD (thin film diode), or the like can also be used. Since such anelement has a small number of manufacturing steps, manufacturing costcan be reduced or yield can be improved. Further, since the size of theelement is small, the aperture ratio can be improved, so that powerconsumption can be reduced or higher luminance can be achieved.

Note that as a method other than the active matrix method, a passivematrix method in which an active element (a non-linear element) is notused can be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or yield can be improved. Further, since an activeelement (a non-linear element) is not used, the aperture ratio can beimproved, so that power consumption can be reduced or higher luminancecan be achieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Thus, a regionwhich serves as a source and a drain is not referred to as a source or adrain in some cases. In that case, one of the source and the drain mightbe referred to as a first terminal and the other of the source and thedrain might be referred to as a second terminal, for example.Alternatively, one of the source and the drain might be referred to as afirst electrode and the other of the source and the drain might bereferred to as a second electrode. Alternatively, one of the source andthe drain might be referred to as a first region and the other of thesource and the drain might be referred to as a second region.

Note that a transistor may be an element having at least three terminalsof a base, an emitter, and a collector. In this case, in a similarmanner, one of the emitter and the collector might be referred to as afirst terminal and the other of the emitter and the collector might bereferred to as a second terminal.

Note that a gate corresponds to all or some of a gate electrode and agate wiring (also referred to as a gate line, a gate signal line, a scanline, a scan signal line, or the like). A gate electrode corresponds topart of a conductive film which overlaps with a semiconductor whichforms a channel region with a gate insulating film interposedtherebetween. Note that part of the gate electrode overlaps with an LDD(lightly doped drain) region or a source region (or a drain region) withthe gate insulating film interposed therebetween in some cases. A gatewiring corresponds to a wiring for connecting gate electrodes oftransistors to each other, a wiring for connecting gate electrodes ofpixels to each other, or a wiring for connecting a gate electrode toanother wiring.

However, there is a portion (a region, a conductive layer, a wiring, orthe like) which serves as both a gate electrode and a gate wiring. Sucha portion (a region, a conductive layer, a wiring, or the like) may bereferred to as either a gate electrode or a gate wiring. That is, thereis a region where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (region, conductive layer, wiring, or the like) serves as both agate wiring and a gate electrode. Thus, such a portion (a region, aconductive layer, a wiring, or the like) may be referred to as either agate electrode or a gate wiring.

Note that a portion (a region, a conductive layer, a wiring, or thelike) which is formed using the same material as a gate electrode, formsthe same island as the gate electrode, and is connected to the gateelectrode may be referred to as a gate electrode. In a similar manner, aportion (a region, a conductive layer, a wiring, or the like) which isformed using the same material as a gate wiring, forms the same islandas the gate wiring, and is connected to the gate wiring may be referredto as a gate wiring. In a strict sense, such a portion (a region, aconductive layer, a wiring, or the like) does not overlap with a channelregion or does not have a function of connecting the gate electrode toanother gate electrode in some cases. However, there is a portion (aregion, a conductive layer, a wiring, or the like) which is formed usingthe same material as a gate electrode or a gate wiring, forms the sameisland as the gate electrode or the gate wiring, and is connected to thegate electrode or the gate wiring because of specifications or the likein manufacturing. Thus, such a portion (a region, a conductive layer, awiring, or the like) may be referred to as either a gate electrode or agate wiring.

Note that in a multi-gate transistor, for example, a gate electrode isconnected to another gate electrode by using a conductive layer which isformed using the same material as the gate electrode in many cases.Since such a portion (a region, a conductive layer, a wiring, or thelike) is a portion (a region, a conductive layer, a wiring, or the like)for connecting the gate electrode to another gate electrode, the portionmay be referred to as a gate wiring, or the portion may be referred toas a gate electrode because a multi-gate transistor can be considered asone transistor. That is, a portion (a region, a conductive layer, awiring, or the like) which is formed using the same material as a gateelectrode or a gate wiring, forms the same island as the gate electrodeor the gate wiring, and is connected to the gate electrode or the gatewiring may be referred to as either a gate electrode or a gate wiring.In addition, for example, part of a conductive layer which connects thegate electrode and the gate wiring and is formed using a material whichis different from that of the gate electrode or the gate wiring may bereferred to as either a gate electrode or a gate wiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive layer, a wiring, or the like) of a gate electrode or part ofa portion (a region, a conductive layer, a wiring, or the like) which iselectrically connected to the gate electrode.

In the case where a wiring is referred to as a gate wiring, a gate line,a gate signal line, a scan line, a scan signal line, or the like, a gateof a transistor is not connected to the wiring in some cases. In thiscase, the gate wiring, the gate line, the gate signal line, the scanline, or the scan signal line corresponds to a wiring formed in the samelayer as the gate of the transistor, a wiring formed using the samematerial of the gate of the transistor, or a wiring formed at the sametime as the gate of the transistor in some cases. As examples, there area wiring for a storage capacitor, a power supply line, a referencepotential supply line, and the like.

Note that a source corresponds to all or some of a source region, asource electrode, and a source wiring (also referred to as a sourceline, a source signal line, a data line, a data signal line, or thelike). A source region corresponds to a semiconductor region containinga large amount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Therefore, a region containinga small amount of p-type impurities or n-type impurities, namely, an LDD(lightly doped drain) region is not included in the source region. Asource electrode is part of a conductive layer which is formed using amaterial different from that of a source region and is electricallyconnected to the source region. However, a source electrode and a sourceregion are collectively referred to as a source electrode in some cases.A source wiring corresponds to a wiring for connecting source electrodesof transistors to each other, a wiring for connecting source electrodesof pixels to each other, or a wiring for connecting a source electrodeto another wiring.

However, there is a portion (a region, a conductive layer, a wiring, orthe like) which serves as both a source electrode and a source wiring.Such a portion (a region, a conductive layer, a wiring, or the like) maybe referred to as either a source electrode or a source wiring. That is,there is a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive layer, wiring, or the like)serves as both a source wiring and a source electrode. Thus, such aportion (a region, a conductive layer, a wiring, or the like) may bereferred to as either a source electrode or a source wiring.

Note that a portion (a region, a conductive layer, a wiring, or thelike) which is formed using the same material as a source electrode,forms the same island as the source electrode, and is connected to thesource electrode, or a portion (a region, a conductive layer, a wiring,or the like) which connects a source electrode and another sourceelectrode may be referred to as a source electrode. Further, a portionwhich overlaps with a source region may be referred to as a sourceelectrode. In a similar manner, a region which is formed using the samematerial as a source wiring, forms the same island as the source wiring,and is connected to the source wiring may be referred to as a sourcewiring. In a strict sense, such a portion (a region, a conductive layer,a wiring, or the like) does not have a function of connecting the sourceelectrode to another source electrode in some cases. However, there is aportion (a region, a conductive layer, a wiring, or the like) which isformed using the same material as a source electrode or a source wiring,forms the same island as the source electrode or the source wiring, andis connected to the source electrode or the source wiring because ofspecifications or the like in manufacturing. Thus, such a portion (aregion, a conductive layer, a wiring, or the like) may be referred to aseither a source electrode or a source wiring.

For example, part of a conductive layer which connects the sourceelectrode and the source wiring and is formed using a material which isdifferent from that of the source electrode or the source wiring may bereferred to as either a source electrode or a source wiring.

Note that a source terminal corresponds to part of a source region, partof a source electrode, or part of a portion (a region, a conductivelayer, a wiring, or the like) which is electrically connected to thesource electrode.

In the case where a wiring is referred to as a source wiring, a sourceline, a source signal line, a data line, a data signal line, or thelike, a source (a drain) of a transistor is not connected to a wiring insome cases. In this case, the source wiring, the source line, the sourcesignal line, the data line, or the data signal line corresponds to awiring formed in the same layer as the source (the drain) of thetransistor, a wiring formed using the same material of the source (thedrain) of the transistor, or a wiring formed at the same time as thesource (the drain) of the transistor in some cases. As examples, thereare a wiring for a storage capacitor, a power supply line, a referencepotential supply line, and the like.

Note that the same can be said for a drain.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also correspond to alldevices that can function by utilizing semiconductor properties. Inaddition, the semiconductor device corresponds to a device having asemiconductor material.

Note that a display device corresponds to a device having a displayelement. The display device may include a plurality of pixels eachhaving a display element. Note that that the display device may includea peripheral driver circuit for driving the plurality of pixels. Theperipheral driver circuit for driving the plurality of pixels may beformed using the same substrate as the plurality of pixels. The displaydevice may include a peripheral driver circuit provided over a substrateby wire bonding or bump bonding, namely, an IC chip connected by chip onglass (COG) or an IC chip connected by TAB or the like. The displaydevice may include a flexible printed circuit (FPC) to which an IC chip,a resistor, a capacitor, an inductor, a transistor, or the like isattached. Note that the display device may include a printed wiringboard (PWB) which is connected through a flexible printed circuit (FPC)and to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. The display device may include anoptical sheet such as a polarizing plate or a retardation plate. Thedisplay device may include a lighting device, a housing, an audio inputand output device, an optical sensor, or the like.

Note that a lighting device may include a backlight unit, a light guideplate, a prism sheet, a diffusion sheet, a reflective sheet, a lightsource (e.g., an LED or a cold cathode fluorescent lamp), a coolingdevice (e.g., a water cooling device or an air cooling device), or thelike.

Note that a light-emitting device corresponds to a device having alight-emitting element or the like. In the case where a light-emittingdevice includes a light-emitting element as a display element, thelight-emitting device is one of specific examples of a display device.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

Note that a liquid crystal display device corresponds to a displaydevice including a liquid crystal element. Liquid crystal displaydevices include a direct-view liquid crystal display, a projectionliquid crystal display, a transmissive liquid crystal display, areflective liquid crystal display, a transflective liquid crystaldisplay, and the like.

Note that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of signals from a sourcesignal line to pixels (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies signals to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies signals to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike), and the like are also examples of the driving device.

Note that a display device, a semiconductor device, a lighting device, acooling device, a light-emitting device, a reflective device, a drivingdevice, and the like overlap with each other in some cases. For example,a display device includes a semiconductor device and a light-emittingdevice in some cases. Alternatively, a semiconductor device includes adisplay device and a driving device in some cases.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it does not necessarily mean that B is formed indirect contact with A. The description includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that “a layerB is formed on (or over) a layer A”, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., a layer C or alayer D) may be a single layer or a plurality of layers.

In a similar manner, when it is explicitly described that “B is formedabove A”, it does not necessarily mean that B is formed in directcontact with A, and another object may be interposed therebetween. Thus,for example, when it is described that “a layer B is formed above alayer A”, it includes both the case where the layer B is formed indirect contact with the layer A, and the case where another layer (e.g.,a layer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or the layer D.Note that another layer (e.g., a layer C or a layer D) may be a singlelayer or a plurality of layers.

Note that when it is explicitly described that “B is formed on A”, “B isformed over A”, or “B is formed above A”, it includes the case where Bis formed obliquely over/above A.

Note that the same can be said when it is described that “B is formedunder A” or “B is formed below A”.

Note that when an object is explicitly described in a singular form, theobject is preferably singular. Note that the present invention is notlimited to this, and the object can be plural. In a similar manner, whenan object is explicitly described in a plural form, the object ispreferably plural. Note that the present invention is not limited tothis, and the object can be singular.

Note that size, the thickness of layers, or regions in the drawings areexaggerated for simplicity in some cases. Thus, the embodiments of thepresent invention are not limited to such scales illustrated in thedrawings.

Note that the drawings are perspective views of ideal examples, andshapes or values are not limited to those illustrated in the drawings.For example, the following can be included: variation in shape due to amanufacturing technique; variation in shape due to an error; variationin signal, voltage, or current due to noise; variation in signal,voltage, or current due to a difference in timing: or the like.

Note that technical terms are used in order to describe a specificembodiment, example, or the like in many cases. There are no limitationsto terms.

Note that terms which are not defined (including terms used for scienceand technology, such as technical terms or academic parlance) can beused as terms which have meaning equal to general meaning that anordinary person skilled in the art understands. It is preferable thatterms defined by dictionaries or the like be construed as consistentmeaning with the background of related art.

Note that terms such as “first”, “second”, “third”, and the like areused for distinguishing various elements, members, regions, layers, andareas from others. Therefore, the terms such as “first”, “second”,“third”, and the like do not limit the number of the elements, members,regions, layers, areas, or the like. Further, for example, “first” canbe replaced with “second”, “third”, or the like.

Note that terms for describing spatial arrangement, such as “over”,“above”, “under”, “below”, “laterally”, “right”, “left”, “obliquely”,“behind”, and “front” are often used for briefly showing a relationshipbetween an element and another element or between a feature and anotherfeature with reference to a diagram. Note that the embodiments of thepresent invention are not limited to this, and such terms for describingspatial arrangement can indicate not only the direction illustrated in adiagram but also another direction. For example, when it is explicitlydescribed that “B is over A”, it does not necessarily mean that B isplaced over A, and can include the case where B is placed under Abecause a device in a diagram can be inverted or rotated by 180°.Accordingly, “over” can refer to the direction described by “under” inaddition to the direction described by “over”. Note that the embodimentsof the present invention are not limited to this, and “over” can referto any of the other directions described by “laterally”, “right”,“left”, “obliquely”, “behind”, and “front” in addition to the directionsdescribed by “over” and “under” because the device in the diagram can berotated in a variety of directions.

In one embodiment of the present invention, a light-transmittingtransistor or a light-transmitting capacitor can be formed. Therefore,even in the case where a transistor or a capacitor is provided in apixel, the aperture ratio can be improved because light can betransmitted also in a portion where the transistor and the capacitor areformed. Further, since a wiring for connecting the transistor and anelement (e.g., a different transistor) to each other or a wiring forconnecting the capacitor and an element (e.g., a different capacitor)can be formed using a material having low resistivity and high electricconductivity, waveform distortion of a signal can be suppressed andvoltage drop due to wiring resistance can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIGS. 2A to 2H are cross-sectional views according to one embodiment ofthe present invention;

FIGS. 3A to 3F are cross-sectional views according to one embodiment ofthe present invention;

FIGS. 4A to 4F are cross-sectional views according to one embodiment ofthe present invention;

FIGS. 5A to 5F are cross-sectional views according to one embodiment ofthe present invention;

FIGS. 6A to 6C are cross-sectional views according to one embodiment ofthe present invention;

FIGS. 7A to 7C are a top view and cross-sectional views according to oneembodiment of the present invention;

FIGS. 8A to 8C are a top view and cross-sectional views according to oneembodiment of the present invention;

FIG. 9 is a top view according to one embodiment of the presentinvention;

FIGS. 10A and 10B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIGS. 11A and 11B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIG. 12 is a top view according to one embodiment of the presentinvention;

FIGS. 13A and 13B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIGS. 14A and 14B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIG. 15 is a top view according to one embodiment of the presentinvention;

FIGS. 16A and 16B are a top view and a cross-sectional view according toone embodiment of the present invention;

FIGS. 17A to 17F are cross-sectional views according to one embodimentof the present invention;

FIGS. 18A to 18F are cross-sectional views according to one embodimentof the present invention;

FIGS. 19A to 19D are cross-sectional views according to one embodimentof the present invention;

FIGS. 20A to 20F are cross-sectional views according to one embodimentof the present invention;

FIGS. 21A to 21D are cross-sectional views according to one embodimentof the present invention;

FIGS. 22A-1 to 22B-2 illustrate multi-tone masks;

FIGS. 23A to 23C are cross-sectional views according to one embodimentof the present invention;

FIGS. 24A and 24B are block diagrams according to one embodiment of thepresent invention;

FIGS. 25A and 25B are cross-sectional views according to one embodimentof the present invention;

FIGS. 26A and 26B are circuit diagrams of a semiconductor deviceaccording to one embodiment of the present invention;

FIGS. 27A to 27C are cross-sectional views of display devices accordingto one embodiment of the present invention;

FIGS. 28A and 28B are a top view and a cross-sectional view of a displaydevice according to one embodiment of the present invention;

FIGS. 29A-1 to 29B are top views and a cross-sectional view according toone embodiment of the present invention;

FIG. 30 is a diagram illustrating a display device according to oneembodiment of the present invention;

FIG. 31 is a diagram illustrating an electronic device according to oneembodiment of the present invention;

FIGS. 32A to 32D are diagrams illustrating electronic devices accordingto one embodiment of the present invention;

FIGS. 33A and 33B are diagrams illustrating electronic devices accordingto one embodiment of the present invention;

FIGS. 34A and 34B are diagrams illustrating electronic devices accordingto one embodiment of the present invention;

FIGS. 35A and 35B are cross-sectional views according to one embodimentof the present invention;

FIG. 36 is a top view according to one embodiment of the presentinvention;

FIGS. 37A to 37G are diagrams illustrating circuits according to oneembodiment of the present invention;

FIGS. 38A to 38D are diagrams illustrating circuits according to oneembodiment of the present invention;

FIGS. 39A to 39D are diagrams illustrating circuits according to oneembodiment of the present invention;

FIGS. 40A to 40F are diagrams showing potentials of a display elementaccording to one embodiment of the present invention; and

FIGS. 41A to 41C are diagrams showing display screens according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Note that the present inventionis not limited to the following description. It will be readilyappreciated by those skilled in the art that modes and details can bechanged in various ways without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the following description of theembodiments.

In this specification, a film refers to what is formed over an entiresurface and is not patterned. In addition, a layer refers to what ispatterned into a desired shape with a resist mask or the like. Note thatthis distinction between “film” and “layer” is for convenience, and theyare used without any distinction in some cases. Also as for each layerof a stacked film, a film and a layer are used without any distinctionin some cases.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used for convenience in order to distinguishelements and do not limit the number, arrangement, and the order ofsteps.

Note that a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in theembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a text described in this specification.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment with another part of the diagram, adifferent diagram (or may be part of the different diagram) illustratedin the embodiment, and/or a diagram (or may be part of the diagram)illustrated in one or a plurality of different embodiments, much morediagrams can be formed.

Note that in a diagram or a text described in one embodiment, part ofthe diagram or the text is taken out, and one embodiment of theinvention can be constituted. Thus, in the case where a diagram or atext related to a certain portion is described, the context taken outfrom part of the diagram or the text is also disclosed as one embodimentof the invention, and one embodiment of the invention can beconstituted.

Therefore, for example, in a diagram (e.g., a cross-sectional view, aplan view, a circuit diagram, a block diagram, a flow chart, a processdiagram, a perspective view, a cubic diagram, a layout diagram, a timingchart, a structure diagram, a schematic view, a graph, a list, a raydiagram, a vector diagram, a phase diagram, a waveform chart, aphotograph, or a chemical formula) or a text in which one or more activeelements (e.g., transistors or diodes), wirings, passive elements (e.g.,capacitors or resistors), conductive layers, insulating layers,semiconductor layers, organic materials, inorganic materials,components, substrates, modules, devices, solids, liquids, gases,operating methods, manufacturing methods, or the like are described,part of the diagram or the text is taken out, and one embodiment of theinvention can be constituted.

Embodiment 1

In this embodiment, semiconductor devices and manufacturing stepsthereof are described with reference to FIGS. 1A and 1B, FIGS. 2A to 2H,FIGS. 3A to 3F, FIGS. 4A to 4F, FIGS. 5A to 5F, FIGS. 6A to 6C, FIGS. 7Ato 7C, FIGS. 8A to 8C, FIG. 9, FIGS. 10A and 10B, FIGS. 11A and 11B,FIG. 12, FIGS. 13A and 13B, FIGS. 14A and 14B, and FIG. 15.

FIGS. 1A and 1B illustrate a semiconductor device in this embodiment.FIG. 1A is a top view, and FIG. 1B is a cross-sectional view taken alongline A-B in FIG. 1A.

The semiconductor device illustrated in FIG. 1A includes a pixel portionwhich has a plurality of wirings (e.g., gate wirings and capacitorwirings) disposed in a first direction, a plurality of wirings (e.g.,source wirings) disposed in a second direction, and a plurality oftransistors disposed around intersections of the wirings. Note that thewirings disposed in the first direction and the wirings disposed in thesecond direction are preferably orthogonal to each other. Note that inthis specification, a pixel portion refers to a region surrounded by aplurality of gate wirings and a plurality of source wirings.

A transistor 150 illustrated in FIGS. 1A and 1B includes a semiconductorlayer 103 a over a substrate 100 having an insulating surface,conductive layers 106 a and 106 b which are provided over thesemiconductor layer 103 a and function as a source electrode and a drainelectrode, a gate insulating film 110 which is provided over theconductive layers 106 a and 106 b functioning as the source electrodeand the drain electrode, and a conductive layer 113 a which is providedover the gate insulating film 110 and functions as a gate electrodeprovided between the conductive layers 106 a and 106 b. Thus, thetransistor 150 is a so-called top-gate transistor. However, the gateelectrode may be provided below a channel (the semiconductor layer 103a). The semiconductor layer 103 a preferably contains an oxide; however,this embodiment is not limited to this. It is acceptable for thesemiconductor layer not to contain an oxide. For example, thesemiconductor layer 103 a can be formed using silicon, gallium arsenide,a compound semiconductor, an organic semiconductor, a carbon nanotube,or the like.

Further, parts or all of the semiconductor layer 103 a, the conductivelayer 113 a functioning as the gate electrode, the conductive layer 106a and 106 b functioning as the source electrode and the drain electrode,and the like which are included in the transistor 150 are formed usinglight-transmitting materials. By forming parts or all of thesemiconductor layer, the conductive layers, and the like which areincluded in the transistor 150 with the use of light-transmittingmaterials in this manner, light can be transmitted in a portion wherethe transistor is formed. Thus, the aperture ration of the pixel portioncan be improved.

In general, a wiring for connecting elements such as transistors to eachother is formed by extending conductive layers used for a gateelectrode, a source electrode, and a drain electrode, so that the wiringis formed in the same island as the conductive layers in many cases.Accordingly, a wiring for connecting a gate of a transistor to a gate ofa different transistor (such a wiring is referred to as a gate wiring)is formed using the same layer structure or material as a gate electrodeof the transistor in many cases; and a wiring for connecting a source ofthe transistor to a source of the different transistor (such a wiring isreferred to as a source wiring) is formed using the same layer structureor material as a source electrode of the transistor in many cases.Therefore, in the case where the gate electrode, the source electrode,and the drain electrode are formed using light-transmitting materials,the gate wiring and the source wiring are formed usinglight-transmitting materials, like the gate electrode, the sourceelectrode, and the drain electrode.

As compared to a material having light-blocking properties andreflecting properties, such as aluminum, molybdenum, titanium, tungsten,neodymium, copper, silver, or chromium, a light-transmitting materialsuch as indium tin oxide, indium zinc oxide, or indium tin zinc oxidetends to have lower electric conductivity. Accordingly, when a wiring isformed using a light-transmitting material, wiring resistance becomeshigh. For example, in the case where a large display device ismanufactured, wiring resistance becomes extremely high because a wiringis long. As wiring resistance increases, the waveform of a signal whichis transmitted through the wiring is distorted, so that voltage which issupplied is lowered by voltage drop due to the wiring resistance.Therefore, it is difficult to supply accurate voltage and current, sothat it might be difficult to perform normal display and operation.

Thus, a gate wiring which is electrically connected to the gateelectrode of the transistor 150 is formed by stacking thelight-transmitting conductive layer 113 a and a light-blockingconductive layer 116 a. In addition, a source wiring which iselectrically connected to the source electrode or the drain electrode ofthe transistor 150 is formed by stacking the light-transmittingconductive layer 106 a and a light-blocking conductive layer 109 a. Thatis, the gate electrode of the transistor 150 is formed using part of thelight-transmitting conductive layer 113 a. Further, the source electrodeor the drain electrode of the transistor 150 is formed using part of thelight-transmitting conductive layer 106 a.

It is preferable that the transmittance of the conductive layer 113 a besufficiently high. Further, the transmittance of the conductive layer113 a is preferably higher than the transmittance of the conductivelayer 116 a.

It is preferable that the resistivity of the conductive layer 116 a besufficiently low and the electric conductivity of the conductive layer116 a be sufficiently high. In addition, the resistivity of theconductive layer 116 a is preferably lower than the resistivity of theconductive layer 113 a. Note that since the conductive layer 116 afunctions as a conductive layer, the resistivity of the conductive layer116 a is preferably lower than the resistivity of an insulating layer.

When the gate wiring or the source wiring is formed by stacking thelight-transmitting conductive layer and the light-blocking conductivelayer, wiring resistance can be lowered. In addition, by lowering thewiring resistance, waveform distortion of a signal can be suppressed andvoltage drop due to the wiring resistance can be reduced. Further, byreducing the voltage drop due to the wiring resistance, accurate voltageand current can be supplied. Thus, a large display device can bemanufactured. Furthermore, since the gate wiring or the source wiring isformed using the light-blocking conductive layer, a space between pixelscan be shielded from light. That is, with the gate wiring disposed in arow direction and the source wiring disposed in a column direction, thespace between the pixels can be shielded from light without use of ablack matrix. However, a black matrix can be used.

In addition, in terms of display performance, large capacitors andhigher aperture ratio are demanded for pixels. Pixels each having highaperture ratio improve light use efficiency, so that power saving andminiaturization of a display device can be achieved. In recent years,the size of pixels has been made smaller and higher-definition imageshave been demanded. However, the decrease in the size of pixels resultsin a large formation area for transistors and wirings, which occupiesone pixel, so that the aperture ratio of the pixel is lowered. Thus, inorder to obtain high aperture ratio in each pixel in a specified size,it is necessary to lay out components needed for the circuit structureof the pixel efficiently.

A capacitor wiring according to one embodiment of the present inventionis disposed in the same direction as the gate wiring and is preferablyformed using a light-transmitting conductive layer 113 b in the pixelregion. In addition, part of the capacitor wiring, which overlaps withthe source wiring, may be formed by stacking the light-transmittingconductive layer 113 b and a light-blocking conductive layer 116 b inorder to increase electric conductivity. Further, a storage capacitorportion 160 is formed in the capacitor wiring. The storage capacitorportion 160 is connected to one of the source electrode and the drainelectrode of the transistor 150 (the conductive layer 106 b). Thestorage capacitor portion 160 includes the gate insulating film 110 as adielectric, and the conductive layers 106 b and 113 b functioning as theelectrodes. Note that since a capacitor is also formed between a pixelelectrode and the conductive layer 113 b, the capacitor also can be usedas a storage capacitor.

In this embodiment, an example is described in which the width of thecapacitor wiring and the width of the gate wiring are the same; however,the width of the capacitor wiring and the width of the gate wiring maybe different from each other. The width of the capacitor wiring ispreferably larger than the width of the gate wiring. When the width ofthe capacitor wiring is made larger, the area of the storage capacitorportion 160 can be increased.

By forming the storage capacitor portion 160 with the use of thelight-transmitting conductive layers 106 b and 113 b as described above,light can be transmitted also in a portion where the storage capacitorportion 160 is formed. Thus, the aperture ratio can be improved. Inaddition, by forming the storage capacitor portion 160 with the use ofthe light-transmitting conductive layers, the storage capacitor portion160 can be made larger without a decrease in the aperture ratio. Thus,even when the transistor is turned off, potential holding properties ofthe pixel electrode is improved and display quality is improved.Further, a feedthrough potential can be lowered. Alternatively, sincenoise immunity is improved, crosstalk can be reduced. Further, sinceaccurate voltage can be supplied, flickers can be reduced. Efficientlayout of circuit components needed for the circuit structure of thepixel is possible.

Further, the transistor 150 illustrated in FIGS. 1A and 1B can be usedas a pixel transistor provided in a pixel portion of a liquid crystaldisplay device or a light-emitting display device typified by an ELdisplay device. Therefore, in FIGS. 1A and 1B, a contact hole 130 isprovided in the gate insulating film 110 and an insulating film 117;pixel electrode layers (light-transmitting conductive layers 119 a and119 c) are provided over the insulating film 117; and the pixelelectrode layer (the light-transmitting conductive layer 119 a) and theconductive layer 106 b are connected to each other through the contacthole 130 provided in the gate insulating film 110 and the insulatingfilm 117.

Next, an example of a manufacturing process of a semiconductor device isdescribed with reference to FIGS. 2A to 2H, FIGS. 3A to 3F, FIGS. 4A to4F, and FIGS. 5A to 5F.

First, an oxide semiconductor film 101 is formed over the substrate 100having an insulating surface (see FIGS. 2A and 2B).

As the substrate 100 having an insulating surface, for example, a glasssubstrate which has visible-light-transmitting properties and is used ina liquid crystal display device or the like can be used. The glasssubstrate is preferably an alkali-free glass substrate. The non-alkaliglass substrate is formed using a glass material such as aluminosilicateglass, aluminoborosilicate glass, or barium borosilicate glass, forexample. Alternatively, as the substrate 100 having an insulatingsurface, an insulating substrate formed using an insulator, such as aceramic substrate, a quartz substrate, or a sapphire substrate; asemiconductor substrate which is formed using a semiconductor materialsuch as silicon and has a surface covered with an insulating material; aconductive substrate which is formed using a conductor such as metal orstainless steel and has a surface covered with an insulating material;or the like can be used. Note that a plastic substrate formed usingpolyethylene terephthalate (PET) or the like can be used.

An insulating film which serves as a base film may be provided over thesubstrate 100 having an insulating surface. The insulating film has afunction of preventing diffusion of an impurity such as an alkali metal(e.g., Li, Cs, or Na), an alkaline earth metal (e.g., Ca or Mg), or adifferent metal element from the substrate 100. Note that theconcentration of Na is 5×10¹⁹/cm³ or lower, preferably 1×10¹⁸/cm³ orlower. The insulating film can be formed to have a single-layerstructure of a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, or an aluminumnitride oxide film; or a layered structure of any of the above films. Asilicon oxide film is preferably provided over a silicon nitride film.With the silicon nitride film, diffusion of an impurity can be preventedsufficiently. Further, by providing a silicon oxide film thereover, thesilicon nitride film can be prevented from being in contact with thesemiconductor layer. This is because if the silicon nitride film is incontact with the semiconductor layer, the semiconductor layer might behydrogenated. Note that this embodiment is not limited to this, and thesilicon nitride film can be in contact with the semiconductor layer.

As an oxide semiconductor contained in the oxide semiconductor film 101,it is preferable to use an oxide semiconductor whose structural formulais represented by InMO₃(ZnO)_(m) (m>0). In particular, it is preferableto use an In-Ga—Zn—O-based oxide semiconductor. Note that M is one ormore metal elements selected from gallium (Ga), iron (Fe), nickel (Ni),manganese (Mn), or cobalt (Co). For example, M is Ga in some cases;meanwhile, Ga and the above metal element other than Ga, such as Ni orFe, are contained in some cases. Further, in the oxide semiconductor, insome cases, a transitional metal element such as Fe or Ni or an oxide ofthe transitional metal is contained as an impurity element in additionto the metal element contained as M. In this specification, among theoxide semiconductors whose structural formulas are represented byInMO₃(ZnO)_(m) (m>0), an oxide semiconductor whose structural formulaincludes at least Ga as M is referred to as an In-Ga—Zn—O-based oxidesemiconductor, and a thin film containing the In-Ga—Zn—O-based oxidesemiconductor is also referred to as an In-Ga—Zn—O-basednon-single-crystal film.

By X-ray diffraction (XRD) spectrometry, an amorphous structure isobserved as the crystal structure of the In-Ga—Zn-O basednon-single-crystal film. Note that the In-Ga—Zn—O-basednon-single-crystal film used as the sample in spectrometry is subjectedto heat treatment at 200 to 500° C., typically 300 to 400° C., for 10 to100 minutes after the film is formed by sputtering.

By using the In-Ga—Zn—O-based non-single-crystal film for an activelayer of a thin film transistor, a thin film transistor havingelectrical characteristics of an on/off ratio of 10⁹ or more and amobility of 10 cm²/V·s or more at a gate voltage of ±20 V can be formed.

However, the oxide semiconductor film 101 is not limited to the oxidesemiconductor film whose structural formula is represented byInMO₃(ZnO)_(m) (m>0). For example, an oxide semiconductor filmcontaining indium oxide (InO_(x)), zinc oxide (ZnO_(x)), tin oxide(SnO), indium zinc oxide (IZO), indium tin oxide (ITO), indium tin oxidecontaining silicon oxide (ITSO), zinc oxide to which gallium is added(GZO), or the like may be used.

The thickness of the oxide semiconductor film 101 is 50 nm or more,preferably 60 to 150 nm. Further, the oxide semiconductor film 101 mightinclude a region whose thickness is smaller than those of regions whereparts of the oxide semiconductor film 101 overlap with the conductivelayers 106 a and 106 b. The region whose thickness is smaller isprovided between the conductive layers 106 a and 106 b which are formedlater and function as the source electrode and the drain electrode. Thisregion is generated because part of the semiconductor layer 103 a isetched when the conductive layers 106 a and 106 b are etched. Thus, whenthe thickness of the oxide semiconductor film 101 is 50 nm or more, achannel formation region can be prevented from being etched away.

The range of the carrier concentration of the oxide semiconductor film101 is preferably lower than 1×10′⁷/cm³ (more preferably 1×10¹¹/cm³ orhigher). When the carrier concentration of the oxide semiconductor film101 exceeds the above range, the thin film transistor might be normallyon.

An insulating impurity may be contained in the oxide semiconductor film101. As the impurity, an insulating oxide typified by silicon oxide,germanium oxide, aluminum oxide, or the like; an insulating nitridetypified by silicon nitride, aluminum nitride, or the like; or aninsulating oxynitride such as silicon oxynitride or aluminum oxynitrideis used.

The insulating oxide, the insulating nitride, or the insulatingoxynitride is added to the oxide semiconductor at a concentration atwhich an electrical conducting property of the oxide semiconductor doesnot deteriorate.

When the insulating impurity is contained in the oxide semiconductorfilm 101, crystallization of the oxide semiconductor film 101 can besuppressed. By suppressing the crystallization of the oxidesemiconductor film 101, characteristics of the thin film transistor canbe stabilized.

For example, when an impurity such as silicon oxide is contained in theIn-Ga—Zn—O-based oxide semiconductor, crystallization of the oxidesemiconductor or generation of microcrystal grains can be prevented evenwhen heat treatment at 300 to 600° C. is performed.

In the manufacturing process of a thin film transistor in which anIn-Ga—Zn—O-based oxide semiconductor is used for its channel formationregion, an S value (a subthreshold swing value) or field effect mobilitycan be improved by heat treatment. Even in such a case, the thin filmtransistor can be prevented from being normally on. Further, even in thecase where heat stress or bias stress is applied to the thin filmtransistor, variations in the threshold voltage can be prevented.

As well as the above oxide semiconductors, any of the following oxidesemiconductors can be used as the oxide semiconductor used for the oxidesemiconductor film 101: an In—Sn—Zn—O-based oxide semiconductor; anIn—Al—Zn—O-based oxide semiconductor; a Sn-Ga—Zn—O-based oxidesemiconductor; an Al-Ga—Zn—O-based oxide semiconductor; aSn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxidesemiconductor; a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-basedoxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; and a Zn—O-based oxide semiconductor. Further, byaddition of an impurity which suppresses crystallization to keep anamorphous state to these oxide semiconductors, characteristics of thethin film transistor can be stabilized.

A semiconductor layer used in one embodiment of the present inventionmay have light-transmitting properties. As well as an oxidesemiconductor, any of a crystalline semiconductor (a single crystalsemiconductor or a polycrystalline semiconductor), an amorphoussemiconductor, a microcrystalline semiconductor, an organicsemiconductor, and the like may be used.

Note that in the case where the insulating film is formed over thesubstrate 100, plasma treatment may be performed on a surface of theinsulating film before the oxide semiconductor film 101 is formed. Byperforming plasma treatment, dust (e.g., a particle) attached to thesurface of the insulating film can be removed.

Note that when a pulsed direct current (DC) power source is used whenthe plasma treatment is performed, dust can be reduced and the thicknessis made uniform, which is preferable. Further, by forming the oxidesemiconductor film 101 without being exposed to the air after the plasmatreatment is performed, attachment of dust or moisture to an interfacebetween the insulating film and the oxide semiconductor film 101 can besuppressed.

Alternatively, a multi-target sputtering apparatus in which a pluralityof targets formed using different materials can be set may be used as asputtering apparatus. In a multi-target sputtering apparatus, a stack ofdifferent films can be formed in one chamber, or one film can be formedby sputtering using plural kinds of materials concurrently in onechamber. Alternatively, a method using a magnetron sputtering apparatusin which a magnetic field generating system is provided inside a chamber(magnetron sputtering), ECR sputtering in which plasma generated byusing a microwave is used, or the like may be employed. Alternatively,reactive sputtering in which a target substance and a sputtering gascomponent chemically react with each other to form a compound thereof atthe time of deposition, bias sputtering in which voltage is applied alsoto a substrate at the time of deposition, or the like may be employed.

Next, a resist mask 102 is formed over the oxide semiconductor film 101and the oxide semiconductor film 101 is selectively etched using theresist mask 102, so that the island-shaped semiconductor layer 103 a isformed (see FIGS. 2C and 2D). In the case of forming the resist mask byspin coating, large quantities of resist materials and a large amount ofdeveloping solutions are used in order to improve the uniformity of aresist film; thus, large quantities of surplus materials are consumed.In a deposition method using spin coating, an increase in the size of asubstrate is particularly disadvantageous in mass production because amechanism for rotating a large substrate is made larger and a loss andwaste amount of a material liquid are large. Further, when a film isformed by spin-coating a rectangular substrate, circular unevenness witha rotating axis as a center is likely to appear in the film. Thus, it ispreferable that a resist material film be selectively formed by a screenprinting method or a droplet discharge method such as an ink jet method,and then a resist mask be formed through exposure. By forming the resistmaterial film selectively, cost can be greatly reduced because reductionin consumption of the resist material can be achieved, and such a largesubstrate having a size of 1000 mm×1200 mm, 1100 mm×1250 mm, or 1150mm×1300 mm can be used. However, this embodiment is not limited to this.

Wet etching or dry etching can be used as etching in this case. Here, anunnecessary portion of the oxide semiconductor film 101 is removed bywet etching using a mixed solution of acetic acid, nitric acid, andphosphoric acid, so that the island-shaped semiconductor layer 103 a isformed. Note that after the etching, the resist mask 102 is removed.Further, an etchant used for the wet etching is not limited to the aboveas long as it can etch the oxide semiconductor film 101. When dryetching is performed, a gas containing chlorine or a gas containingchlorine, to which oxygen is added, is preferably used. This is because,by using a gas containing chlorine and oxygen, the etching selectivityof the oxide semiconductor film 101 to the insulating film serving asthe base film is likely to be high, and the insulating film can besufficiently prevented from being damaged.

In addition, as an etching apparatus used for the dry etching, anetching apparatus using reactive ion etching (RIE) or a dry etchingapparatus using a high-density plasma source such as ECR (electroncyclotron resonance) or ICP (inductively coupled plasma) can be used.Furthermore, as a dry etching apparatus by which electric discharge islikely to be homogeneous in a large area as compared to an ICP etchingapparatus, there is an ECCP (enhanced capacitively coupled plasma) modeetching apparatus in which an upper electrode is grounded, ahigh-frequency power source of 13.56 MHz is connected to a lowerelectrode, and a low-frequency power source of 3.2 MHz is connected tothe lower electrode. This ECCP mode etching apparatus can be applied,for example, even when a substrate of the tenth generation with a sizelarger than 3 m is used.

After that, heat treatment is preferably performed at 200 to 600° C.,typically 300 to 500° C. Here, heat treatment is performed at 350° C.for 1 hour in a nitrogen atmosphere. Through this heat treatment,rearrangement at the atomic level is caused in the In-Ga—Zn—O-basedoxide semiconductor included in the semiconductor layer 103 a. This heattreatment (including light annealing or the like) is important becausethe strain which inhibits the movement of carriers in the semiconductorlayer 103 a can be released. Note that timing at which the heattreatment is performed is not particularly limited to certain timing aslong as it is after the formation of the semiconductor layer 103 a.

Next, a conductive film 104 is formed over the island-shapedsemiconductor layer 103 a (see FIGS. 2E and 2F).

As the conductive film 104, indium tin oxide (ITO), indium tin oxidecontaining silicon oxide (ITSO), organoindium, organotin, zinc oxide(ZnO), titanium nitride, or the like can be used. Alternatively, indiumzinc oxide (IZO) containing zinc oxide, zinc oxide containing gallium(Ga), tin oxide (SnO₂), indium oxide containing tungsten oxide, indiumzinc oxide containing tungsten oxide, indium oxide containing titaniumoxide, indium tin oxide containing titanium oxide, or the like may beused. The conductive film 104 can be formed to have a single-layerstructure or a layered structure of such a material by sputtering. Notethat in the case of the layered structure, the transmittance of each ofa plurality of films is preferably high enough.

Next, resist masks 105 a and 105 b are formed over the conductive film104 and the conductive film 104 is selectively etched using the resistmasks 105 a and 105 b, so that the conductive layers 106 a and 106 bfunctioning as the source electrode and the drain electrode are formed(see FIGS. 2G and 2H). Note that the resist masks 105 a and 105 b areremoved after the etching. In this case, in order to improve coveragewith the gate insulating film 110 which is formed later and to preventbreakage, the etching is preferably performed so that end portions ofthe conductive layers 106 a and 106 b functioning as the sourceelectrode and the drain electrode are tapered. Note that the sourceelectrode or the drain electrode includes the electrode and the wiringformed using the conductive film, such as the source wiring.

Next, a conductive film 107 is formed over the island-shapedsemiconductor layer 103 a and the conductive layers 106 a and 106 b (seeFIGS. 3A and 3B).

The conductive film 107 can be formed to have a single-layer structureor a layered structure of a metal material such as aluminum (Al),tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel(Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese(Mn), neodymium (Nd), chromium (Cr), antimony (Sb), niobium (Nb), orcerium (Ce); an alloy material containing any of the above metalmaterials as its main component; or a nitride containing any of theabove metal materials as its component. The conductive film 107 ispreferably formed using a low-resistant conductive material such asaluminum.

In the case where the conductive film 107 is formed over the conductivelayers 106 a and 106 b (or the conductive film 104), both the filmsreact with each other in some cases. For example, in the case where theconductive layers 106 a and 106 b are formed using ITO and theconductive film 107 is formed using aluminum, chemical reaction occurstherebetween in some cases. Accordingly, in order to avoid such chemicalreaction, a high-melting point material is preferably used between theconductive layers 106 a and 106 b and the conductive film 107. Forexample, as the high-melting point material, molybdenum, titanium,tungsten, tantalum, chromium, or the like can be used. Further, it ispreferable to form the conductive layers 106 a and 106 b as amulti-layer film by using a material having high electric conductivityover the film formed using the high-melting point material. As thematerial having high electric conductivity, aluminum, copper, silver, orthe like can be used. For example, in the case where the conductivelayers 106 a and 106 b are formed to have a layered structure, a stackof molybdenum as a first layer, aluminum as a second layer, andmolybdenum as a third layer, or a stack of molybdenum as a first layer,aluminum containing a small amount of neodymium as a second layer, andmolybdenum as a third layer can be used. With such a structure,generation of hillocks can be prevented. Note that the thickness of thelight-transmitting conductive layer is preferably smaller than thethickness of the light-blocking conductive layer. However, thisembodiment is not limited to this.

Next, a resist mask 108 is formed over the conductive film 107 and theconductive film 107 is selectively etched using the resist mask 108, sothat the conductive layer 109 a is formed (see FIGS. 3C and 3D). Afterthe etching, the resist mask 108 is removed. Accordingly, part of theconductive film 107, over which the resist mask 108 is not formed, isremoved, so that the conductive layer 106 a is exposed. Thus, thesurface areas of the conductive layer 109 a and the conductive layer 106a are different from each other. That is, the surface area of theconductive layer 106 a is larger than the surface area of the conductivelayer 109 a. Alternatively, as for the conductive layers 109 a and 106a, there are a region where the conductive layers 109 a and 106 aoverlap with each other and a region where the conductive layers 109 aand 106 a do not overlap with each other.

In the region where the conductive layers 109 a and 106 a overlap witheach other, the conductive layers 106 a and 109 a function as the sourcewiring. In the region where the conductive layers 106 a and 109 a do notoverlap with each other, the conductive layer 106 a functions as thesource electrode or the drain electrode. By forming the conductive layer106 a functioning as the source electrode or the drain electrode withthe use of a light-transmitting conductive material, light can betransmitted also in a portion where the source electrode or the drainelectrode is formed; therefore, the aperture ratio of a pixel can beimproved. In addition, by forming the conductive layer 109 a with theuse of a material which has higher electric conductivity than a materialused for the conductive layer 106 a, wiring resistance of the sourcewiring can be reduced, and power consumption can be reduced. Further,since the source wiring is formed using the light-blocking conductivelayer, a space between pixels can be shielded from light. Further,contrast can be improved.

Note that although the step in which the conductive layer 109 a isformed after the conductive layers 106 a and 106 b are formed isdescribed, the order of formation may be inverted. In other words, afterthe conductive layer 109 a which is part of the source wiring is formed,the conductive layers 106 a and 106 b which function as the sourceelectrode and the drain electrode can be formed (see FIG. 7B).

Further, the conductive layer 106 b functions also as an electrode ofthe storage capacitor portion 160.

Next, after the gate insulating film 110 is formed so as to cover theconductive layers 106 a and 106 b, a conductive film 111 is formed (seeFIGS. 3E and 3F).

The gate insulating film 110 can be formed to have a single layer or astacked layer of a silicon oxide film, a silicon oxynitride film, asilicon nitride film, a silicon nitride oxide film, an aluminum oxidefilm, an aluminum nitride film, an aluminum oxynitride film, an aluminumnitride oxide film, or a tantalum oxide film. The gate insulating film110 can be formed to a thickness of 50 to 250 nm by sputtering, CVD, orthe like. For example, as the gate insulating film 110, a 100-nm-thicksilicon oxide film can be formed by sputtering. Alternatively, a100-nm-thick aluminum oxide film can be formed by sputtering.

By forming the gate insulating film 110 as a dense film, moisture oroxygen can be prevented from entering the semiconductor layer 103 a fromthe substrate 100 side. In addition, an impurity included in thesubstrate 100, such as an alkali metal (e.g., Li, Cs, or Na), analkaline earth metal (e.g., Ca or Mg), or a different metal element, canbe prevented from entering the semiconductor layer 103 a. Note that theconcentration of Na is 5×10¹⁹/cm³ or lower, preferably 1×10¹⁸/cm³ orlower. Therefore, variations in semiconductor properties of thesemiconductor device in which an oxide semiconductor is used can besuppressed. Further, reliability of the semiconductor device can beimproved.

The gate insulating film 110 can be formed to have a single-layerstructure or a layered structure of any of an insulating film containingoxygen and/or nitrogen, such as silicon oxide, silicon nitride, siliconoxynitride, or silicon nitride oxide; a film containing carbon such asDLC (diamond-like carbon); and a film formed using an organic materialsuch as epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene,or acrylic, or a siloxane material such as a siloxane resin.

Note that the gate insulating film 110 preferably has light-transmittingproperties.

The conductive film 111 is preferably formed using a material which issubstantially the same as the material used for the conductive film 104.However, this embodiment is not limited to this. Substantially the samematerial is a material whose element of a main component is the same asthat of the material used for the conductive film 104. In terms ofimpurities, the kind and the concentration of elements contained aredifferent from each other in some cases. In the case where theconductive film 111 is formed using the material which is substantiallythe same as the material used for the conductive film 104 by sputtering,evaporation, or the like in this manner, there is an advantage that thematerial can be shared between the conductive films 111 and 104. In thecase where the material is shared between the conductive films 111 and104, the same manufacturing apparatus can be used, manufacturing stepscan proceed smoothly, and throughput can be improved, which leads toreduction in cost.

Next, resist masks 112 a and 112 b are formed over the conductive film111 and the conductive film 111 is selectively etched using the resistmasks 112 a and 112 b, so that conductive layers 113 a and 113 b areformed (see FIGS. 4A and 4B). Note that after the etching, the resistmasks 112 a and 112 b are removed.

Next, a conductive film 114 is formed over the conductive layers 113 aand 113 b and the gate insulating film 110 (see FIGS. 4C and 4D).

The conductive film 114 can be formed to have a single-layer structureor a layered structure of a metal material such as aluminum (Al),tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel(Ni), platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese(Mn), neodymium (Nd), chromium (Cr), antimony (Sb), niobium (Nb), orcerium (Ce); an alloy material containing any of the above metalmaterials as its main component; or a nitride containing any of theabove metal materials as its component. The conductive film 114 ispreferably formed using a low-resistant conductive material such asaluminum.

Further, the conductive film 114 is preferably formed using a materialwhich is different from the material used for the conductive film 107.Alternatively, the conductive film 114 is preferably formed to have alayered structure which is different from the layered structure of theconductive film 107. This is because in manufacturing steps of thesemiconductor device, temperatures of heat applied to the conductivefilm 114 and the conductive film 107 are different from each other inmany cases. In general, the conductive film 107 tends to have highertemperature. Accordingly, the conductive film 107 is preferably formedusing a material or a layered structure having higher melting point.Alternatively, the conductive film 107 is preferably formed using amaterial or a layered structure in which hillocks are less likely tooccur. Alternatively, since the conductive film 114 is included in asignal line through which a video signal is supplied in some cases, theconductive film 114 is preferably formed using a material or a layeredstructure having wiring resistance lower than the conductive film 107.Note that the thickness of the light-transmitting conductive layer ispreferably smaller than the thickness of the light-blocking conductivelayer.

As in the case where the conductive film 107 is formed over theconductive layers 106 a and 106 b (or the conductive film 104), in thecase where the conductive film 114 is formed over the conductive layers113 a and 113 b (or the conductive film 111), both the films react witheach other in some cases. Accordingly, also in the case where theconductive film 114 is formed over the conductive layers 113 a and 113b, a high-melting point material is preferably used between theconductive layers 113 a and 113 b and the conductive film 114. Forexample, as the high-melting point material, molybdenum, titanium,tungsten, tantalum, chromium, or the like can be used. Further, it ispreferable to form the conductive film 114 as a multi-layer film byusing a material having high electric conductivity over the film formedusing the high-melting point material. As the material having highelectric conductivity, aluminum, copper, silver, or the like can beused.

Next, a resist mask 115 is formed over the conductive film 114 and theconductive film 114 is selectively etched using the resist mask 115, sothat the conductive layer 116 a is formed (see FIGS. 4E and 4F). Afterthe etching, the resist mask 115 is removed. Accordingly, part of theconductive film 114, over which the resist mask 115 is not formed, isremoved, so that the conductive layer 113 a is exposed. Thus, thesurface areas of the conductive layer 116 a and the conductive layer 113a are different from each other. That is, the surface area of theconductive layer 113 a is larger than the surface area of the conductivelayer 116 a. Alternatively, as for the conductive layers 116 a and 113a, there are a region where the conductive layers 116 a and 113 aoverlap with each other and a region where the conductive layers 116 aand 113 a do not overlap with each other.

In the region where the conductive layers 113 a and 116 a overlap witheach other, the conductive layers 113 a and 116 a function as the gatewiring. In the region where the conductive layers 113 a and 116 a do notoverlap with each other, the conductive layer 113 a functions as thegate electrode. By forming the conductive layer 113 a functioning as thegate electrode with the use of a light-transmitting conductive material,light can be transmitted also in a portion where the gate electrode isformed; therefore, the aperture ratio of the pixel can be improved. Inaddition, by forming the conductive layer 116 a functioning as the gatewiring with the use of a material which has higher electric conductivitythan a material used for the conductive layer 113 a, wiring resistancecan be reduced, and power consumption can be reduced. Further, since thesource wiring is formed using the light-blocking conductive layer 116 a,the space between the pixels can be shielded from light. That is, withthe gate wiring disposed in the row direction and the source wiringdisposed in the column direction, the space between the pixels can beshielded from light without use of a black matrix.

Note that although the step in which the conductive layer 116 a isformed after the conductive layers 113 a and 113 b are formed isdescribed, the order of formation may be inverted. In other words, afterthe conductive layer 116 a functioning as the gate wiring is formed, theconductive layer 113 a functioning as the gate electrode can be formed(see FIGS. 7A to 7C).

Further, the capacitor wiring is provided in the same direction as thegate wiring. In a pixel region, the capacitor wiring is preferablyformed using the light-transmitting conductive layer 113 b; however, ina region where part of the capacitor wiring overlaps with the sourcewiring, the conductive layer 113 b and the conductive layer 116 b may bestacked. By stacking the conductive layer 113 b and the conductive layer116 b which has higher electric conductivity than the conductive layer113 b, resistance can be lowered (see FIG. 1A).

In this embodiment, an example is described in which the width of thecapacitor wiring and the width of the gate wiring are the same; however,the width of the capacitor wiring and the width of the gate wiring maybe different from each other. The width of the capacitor wiring ispreferably larger than the width of the gate wiring. The surface area ofthe storage capacitor portion 160 can be increased.

By forming the storage capacitor portion 160 with the use of thelight-transmitting conductive layers as described above, light can betransmitted also in a portion where the storage capacitor portion 160 isformed. Thus, the aperture ratio can be improved. In addition, byforming the storage capacitor portion 160 with the use of thelight-transmitting conductive materials, the storage capacitor portion160 can be made larger. Thus, even when the transistor is turned off,potential holding properties of the pixel electrode is improved anddisplay quality is improved. Further, a feedthrough potential can belowered.

In this manner, the transistor 150 and the storage capacitor portion 160can be formed. Further, the transistor 150 and the storage capacitorportion 160 can be light-transmitting elements.

Note that treatment for increasing electric conductivity in part of orwhole regions of the semiconductor layer 103 a may be performed afterformation of the semiconductor layer 103 a, after formation of thesource electrode and the source wiring, after formation of the gateinsulating film, or after formation of the gate electrode and the gatewiring. For example, hydrogenation treatment or the like can be used asthe treatment for increasing electric conductivity. By providing siliconnitride containing hydrogen over the semiconductor layer 103 a andapplying heat, the semiconductor layer 103 a can be hydrogenated.Alternatively, by applying heat in a hydrogen atmosphere, hydrogenationcan be performed. Alternatively, as illustrated in FIG. 6A, by forming achannel protective layer 120 a in a region overlapping with a channelformation region of the semiconductor layer 103 a of a transistor 151,regions 121 a and 121 b whose electric conductivity is increased can beselectively formed in the semiconductor layer 103 a.

The channel protective layer 120 a is preferably formed using siliconoxide. By forming the channel protective layer 120 a with the use ofsilicon oxide, hydrogen can be prevented from entering the channelformation portion of the semiconductor layer 103 a. Note that thechannel protective layer 120 a may be removed after the treatment forincreasing electric conductivity is performed. Alternatively, thechannel protective layer 120 a can be formed using a resist (see FIG.6B). In this case, the resist is preferably removed after hydrogenationtreatment. By performing treatment for increasing electric conductivityon an oxide semiconductor layer as described above, current can flowthrough a transistor easily, and resistance of a capacitor can belowered.

Although FIG. 6A illustrates an example in which the channel protectivelayer 120 a of the transistor 151 is formed in contact with thesemiconductor layer 103 a, the channel protective layer 120 a may beprovided over the gate insulating film 110. Further, by adjusting theshapes of the channel protective layer and the conductive layerfunctioning as the gate electrode so that the channel protective layeris larger than the conductive layer, an offset region can be formed.

With the channel protective layer 120 a, the semiconductor layer 103 acan be prevented from being etched when the conductive layers 106 a and106 b are etched. Thus, the thickness of the semiconductor layer 103 acan be made smaller. When the semiconductor layer 103 a is thin, adepletion layer is easily formed. Therefore, a subthreshold swing valuecan be decreased. The amount of off-state current can be made smaller.

Alternatively, as illustrated in FIG. 6C, a transistor 152 having theregions 121 a and 121 b whose electric conductivity is made higher thanthat of the semiconductor layer 103 a can be provided over thesemiconductor layer 103 a.

Next, after the insulating film 117 is formed, a resist mask (notillustrated) is formed over the insulating film 117 and the insulatingfilm 117 is etched using the resist mask, so that the contact hole 130is formed in the insulating film 117 (see FIGS. 5A and 5B). Theinsulating film 117 functions as an insulating film for flattening asurface over which the transistor 150, the storage capacitor portion160, the wirings, and the like are formed. Since the transistor 150 andthe storage capacitor portion 160 can be formed as light-transmittingelements, regions where they are provided can also be used as openingregions. Therefore, it is advantageous to relieve unevenness due to thetransistor 150, the storage capacitor portion 160, the wirings, or thelike, so that an upper portion over which these elements are formed isflattened.

Further, the insulating film 117 can function as an insulating filmwhich protects the transistor 150 from impurities or the like. Theinsulating film 117 can be formed using, for example, a film containingsilicon nitride. A film containing silicon nitride is preferable becauseit is highly effective in blocking impurities. Alternatively, theinsulating film 117 may be formed using a film containing an organicmaterial. As the organic material, acrylic, polyimide, polyamide, or thelike is preferable. Such organic materials are preferable in terms ofhigh functionality of flattening unevenness. Accordingly, in the casewhere the insulating film 117 is formed to have a layered structure of afilm containing silicon nitride and a film containing an organicmaterial, it is preferable to provide the film containing siliconnitride and the film containing an organic material on the lower sideand on the upper side, respectively. Note that in the case where theinsulating film 117 is formed to have a layered structure, thetransmittance of each of the films is preferably high enough.Alternatively, a photosensitive material can be used. In this case, itis not necessary to etch the insulating film 117 to form a contact hole.

Note that the insulating film 117 may have a function as a color filter.When a color filter is provided on the substrate 100 side, it is notnecessary to provide a color filter on the counter substrate side.Therefore, a margin for adjusting the positions of the two substrates isnot necessary, which can facilitate manufacture of a panel. Note thatthe insulating film 117 is not necessarily formed. The pixel electrodemay be formed over the same layer as the gate electrode and the gatewiring.

Next, a conductive film 118 is formed over the insulating film 117 andthe contact hole 130 (see FIGS. 5C and 5D). The conductive film 118 ispreferably formed using a material which is substantially the same asthe materials of the conductive film 104 and the conductive film 111.When the conductive film 118 is formed using the material which issubstantially the same as the materials of the conductive film 104 andthe conductive film 111 by sputtering or evaporation in this manner,there is an advantage that the material can be shared among theconductive films 104 and 111 and the conductive film 118. When thematerial can be shared, the same manufacturing apparatus can be used,manufacturing steps can proceed smoothly, and throughput can beimproved, which leads to reduction in cost. Note that the conductivefilm 118 may be formed using a material which is different from thematerials of the conductive film 104 and the conductive film 111.

Next, a resist mask (not illustrated) is formed over the conductive film118 and the conductive film 118 is selectively etched using the resistmask, so that conductive layers 119 a, 119 b, and 119 c are formed (seeFIGS. 5E and 5F). Note that the resist mask is removed after theetching.

The conductive layers 119 a, 119 b, and 119 c function as pixelelectrodes. Further, the conductive layers 119 a, 119 b, and 119 c canconnect the source wiring, the source electrode, the gate wiring, thegate electrode, the pixel electrode, the capacitor wiring, the electrodeof the storage capacitor portion, and the like to each other through thecontact hole 130. Therefore, the conductive layers 119 a, 119 b, and 119c can function as wirings for connecting conductors. The thickness ofeach of the conductive layers 119 a, 119 b, and 119 c is preferablysmaller than the thickness of the light-transmitting conductive layerused for the source wiring including the source electrode or thethickness of the light-transmitting conductive layer used for the gatewiring including the gate electrode. However, one embodiment of thepresent invention is not limited to this. The thickness of each of theconductive layers 119 a, 119 b, and 119 c may be larger than thethickness of the light-transmitting conductive layer used for the sourcewiring including the source electrode or the thickness of thelight-transmitting conductive layer used for the gate wiring includingthe gate electrode.

Through the above steps, the semiconductor device shown in FIGS. 1A and1B can be manufactured. By the manufacturing method described in thisembodiment, the light-transmitting transistor 150 and thelight-transmitting storage capacitor portion 160 can be formed.Therefore, even in the case where a transistor or a capacitor isprovided in a pixel, the aperture ratio can be improved because lightcan also be transmitted in a portion where the transistor and thecapacitor are formed. Further, since a wiring for connecting thetransistor and an element (e.g., a different transistor) to each othercan be formed using a material having low resistivity and high electricconductivity, waveform distortion of a signal can be suppressed andvoltage drop due to wiring resistance can be reduced.

Next, a different example of a semiconductor device is described withreference to FIGS. 7A to 7C, FIGS. 8A to 8C, FIG. 9, FIGS. 10A and 10B,FIGS. 11A and 11B, FIG. 12, FIGS. 13A and 13B, FIGS. 14A and 14B, andFIG. 15. Note that many portions are common between the semiconductordevice illustrated in FIGS. 7A to 7C, FIGS. 8A to 8C, FIG. 9, FIGS. 10Aand 10B, FIGS. 11A and 11B, FIG. 12, FIGS. 13A and 13B, FIGS. 14A and14B, and FIG. 15 and the semiconductor device in FIGS. 1A and 1B.Therefore, description of common portions is omitted and differences aredescribed.

FIG. 7A is a plan view, FIG. 7B is a cross-sectional view taken alongline A-B in FIG. 7A, and FIG. 7C is a cross-sectional view taken alongline C-D in FIG. 7A. In FIGS. 1A and 1B, an example is described inwhich the gate wiring and the source wiring are each formed by stackinga light-blocking conductive layer over a light-transmitting conductivelayer; however, the gate wiring and the source wiring may be each formedby stacking a light-blocking conductive layer and a light-transmittingconductive layer in that order (see FIGS. 7A to 7C). Thelight-transmitting conductive layer 113 a functioning as the gateelectrode may be connected to the light-blocking conductive layer 116 afunctioning as the gate wiring. Further, the light-transmittingconductive layer 106 a functioning as the source electrode or the drainelectrode may be connected to the light-blocking conductive layer 109 afunctioning as the source wiring.

FIG. 8A is a plan view, FIG. 8B is a cross-sectional view taken alongline A-B in FIG. 8A, and FIG. 8C is a cross-sectional view taken alongline C-D in FIG. 8A. In FIGS. 1A and 1B, an example is described inwhich the gate wiring and the source wiring are each formed by stackinga light-transmitting conductive layer and a light-blocking conductivelayer in that order; however, the gate wiring and the source wiring maybe each formed using a light-blocking conductive layer (see FIGS. 8A to8C). The light-transmitting conductive layer 113 a functioning as thegate electrode may be connected to the light-blocking conductive layer116 a functioning as the gate wiring. Further, the light-transmittingconductive layer 106 a functioning as the source electrode or the drainelectrode may be connected to the light-blocking conductive layer 109 afunctioning as the source wiring. FIGS. 7A to 7C illustrate an examplein which a light-blocking conductive layer and a light-transmittingconductive layer are stacked in that order, and FIGS. 8A to 8Cillustrate an example in which the gate wiring and the source wiring areeach formed using a light-blocking conductive layer; however, alight-transmitting conductive layer and a light-blocking conductivelayer can be stacked in that order.

In addition, in this embodiment, since the transistor can be formed in apixel, the size of the transistor can be made larger. For example, asillustrated in FIG. 9, a transistor whose channel width W or channellength L is larger than the width of the gate wiring can be formed. Bymaking a transistor larger, its current supply capability can besufficiently high, and a writing time of a signal to the pixel can beshortened. Alternatively, the amount of off-state current can be reducedand flickers or the like can be reduced. Thus, a high-definition displaydevice can be provided.

Note that it is not necessary that light be transmitted through atransistor portion in a protection circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor in a pixel portion may be formed usinglight-transmitting materials and a transistor in the peripheral drivercircuit portion may be formed using a light-blocking material (see FIG.25A).

FIG. 10A is a plan view, and FIG. 10B is a cross-sectional view takenalong line A-B in FIG. 10A. FIGS. 10A and 10B differ from FIGS. 1A and1B in that the surface areas of conductive layers 106 c and 113 c arelarger than the surface areas of the conductive layer 106 b and 113 b.The size of a storage capacitor portion 161 is preferably larger thanthe pixel pitch by 70% or more, or 80% or more. Further, the storagecapacitor portion 161 is in contact with the pixel electrode over aconductive layer 109 b provided over the conductive layer 106 c. Sincethe structure is similar to the structure in FIGS. 1A and 1B, detaileddescription thereof is omitted.

With such a structure, the large storage capacitor portion 161 with hightransmittance can be formed. By forming the large storage capacitorportion 161, even when the transistor is turned off, potential holdingproperties of the pixel electrode is improved and display quality isimproved. Further, a feedthrough potential can be lowered. Furthermore,even in the case where the large storage capacitor portion 161 isformed, light can be transmitted also in a portion where the storagecapacitor portion 161 is formed. Therefore, the aperture ratio can beimproved and power consumption can be reduced. Moreover, even whenmisalignment of liquid crystals is caused by unevenness due to thecontact hole in the pixel electrode, light leakage can be prevented bythe light-blocking conductive layer 109 b.

FIG. 11A is a plan view, and FIG. 11B is a cross-sectional view takenalong line A-B in FIG. 11A.

In a semiconductor device illustrated in FIGS. 11A and 11B, a regionwith high electric conductivity (also referred to as an n⁺ region) isprovided in part of the semiconductor layer 103 a, and the conductivelayers 106 a and 106 b functioning as the source electrode and the drainelectrode are provided so as not to overlap with the gate electrode. Inthe semiconductor layer 103 a, the region with high electricconductivity can be provided in a region which is connected to theconductive layers 106 a and 106 b. Note that the region with highelectric conductivity may be provided so as to overlap with the gateelectrode (the conductive layer 113 a) or so as not to overlap with thegate electrode (the conductive layer 113 a).

The region with high electric conductivity can be formed by selectiveaddition of hydrogen to the semiconductor layer 103 a, as illustrated inFIG. 6B. Hydrogen may be added to a portion of the semiconductor layer103 a, whose electric conductivity is to be made higher.

Further, when the source electrode and the drain electrode are providedso as not to overlap with the gate electrode, parasitic capacitancegenerated between the source electrode and the drain electrode, and thegate electrode can be reduced. Therefore, feedthrough can be reduced.

In FIGS. 11A and 11B, the electric conductivity of part of thesemiconductor layer 103 a is made higher. With such a structure, in atransistor 154, it not necessary to overlap the gate electrode with thesource electrode or the drain electrode.

Note that although each of the source wiring and the gate wiring has astack of a light-blocking conductive layer and a light-transmittingconductive layer, this embodiment is not limited to this. Each of thesource wiring and the gate wiring may be formed using only alight-blocking conductive layer or a light-transmitting conductivelayer. For example, FIG. 12 illustrates the case where the gate wiringis formed using only a light-blocking conductive layer, the sourcewiring is formed using only a light-blocking conductive layer, and thedrain electrode is formed using a light-transmitting conductive layer.The source wiring is formed using only a light-blocking conductivelayer, and the gate wiring is formed using only a light-blockingconductive layer. The capacitor wiring may be formed using either alight-blocking conductive layer or a light-transmitting layer. Note thatin a region where the light-transmitting conductive layer used for thesource electrode and the gate wiring overlap with each other, alight-blocking conductive layer may be formed.

In FIG. 13A and FIG. 14A, pixel structures of light-emitting displaydevices are described as examples of pixel structures. A pixelillustrated in FIG. 13A includes a gate wiring formed by stacking theconductive layer 106 a and the conductive layer 109 a in that order, asource wiring formed by stacking the conductive layer 119 a and theconductive layer 116 a in that order, the switching transistor 150, adriving transistor 155, a storage capacitor portion 162, and a powersupply line formed by stacking a conductive layer 106 d and a conductivelayer 109 c in that order. Further, a pixel illustrated in FIG. 14Aincludes a gate wiring formed by stacking the conductive layer 106 a andthe conductive layer 109 a in that order, a source wiring formed bystacking the conductive layer 113 a and the conductive layer 116 a inthat order, the switching transistor 150, a driving transistor 156, astorage capacitor portion 164, and a power supply line formed bystacking the conductive layer 106 d and the conductive layer 109 c inthat order.

The transistor 150 illustrated in FIGS. 13A and 14A includes thesemiconductor layer 103 a over the substrate 100 having an insulatingsurface, the conductive layers 106 a and 106 c which are provided overthe semiconductor layer 103 a and function as the source electrode andthe drain electrode, the gate insulating film 110 which is provided overthe conductive layers 106 a and 106 c, and the conductive layer 113 awhich is provided over the gate insulating film 110 and functions as thegate electrode provided between the conductive layers 106 a and 106 c.In addition, each of the driving transistors 155 and 156 includes asemiconductor layer 103 b over the substrate 100 having an insulatingsurface, the conductive layer 106 d and a conductive layer 106 e whichare provided over the semiconductor layer 103 b and function as a sourceelectrode and a drain electrode, the gate insulating film 110 which isprovided over the conductive layers 106 d and 106 e, and the conductivelayer 113 c or a conductive layer 113 d which is provided over the gateinsulating film 110 and functions as a gate electrode provided betweenthe conductive layers 106 d and 106 e. Further, the storage capacitorportion 162 includes the conductive layer 106 e and the conductive layer113 c in FIGS. 13A and 13B, and the storage capacitor portion 164includes the conductive layer 106 e and the conductive layer 113 d inFIGS. 14A and 14B.

Note that in the case where a gate and a drain are connected to eachother as illustrated in FIG. 13B, the gate and the drain are connectedto each other through contact holes 132 and 133 via ITO which isprovided at the top; however, the gate and the drain may be directlyconnected through a contact hole 131 as illustrated in FIG. 14B. In thiscase, the area of a pixel electrode can be increased, so that theaperture ratio can be improved. Further, a resistance value can bedecreased.

Although the semiconductor device illustrated in FIGS. 13A and 13B andFIGS. 14A and 14B includes two transistors: the switching transistor 150and the driving transistor 155 or 156, three or more transistors may beprovided in one pixel.

In one embodiment of the present invention, even in the case where twoor more transistors are provided in one pixel as described above, lightcan be transmitted also in portions where the transistors are formed.Therefore, the aperture ratio can be improved.

FIG. 15 is a plan view in the case where the shape of the transistor isa shape in which the conductive layer 106 a surrounds the conductivelayer 106 b (for example, a U-shape or a C-shape).

The transistor 156 illustrated in FIG. 15 includes a semiconductor layer103 c over the substrate 100 having an insulating surface, theconductive layers 106 a and 106 b which are provided over thesemiconductor layer 103 c and function as the source electrode and thedrain electrode, the gate insulating film 110 which is provided over theconductive layers 106 a and 106 b, and the conductive layer 113 a whichis provided over the gate insulating film 110 and functions as a gateelectrode. In the case where one of the source electrode and the drainelectrode surrounds the other of the source electrode and the drainelectrode (for example, the former electrode is in a U-shape or aC-shape) in this manner, a distance between the source electrode and thedrain electrode is kept substantially constant.

When the transistor 156 has the above shape, the channel width of thetransistor can be increased and the area of a region through whichcarries transfer can be increased. Thus, the amount of current can beincreased and the area of the transistor can be decreased. In addition,variations in electrical characteristics can be suppressed.

Note that although a structure where a capacitor wiring is provided isdescribed in this embodiment, a storage capacitor can be providedwithout provision of a capacitor wiring by overlapping a pixel electrodewith a gate wiring provided adjacent to the pixel electrode with aninsulating film interposed therebetween (see FIG. 36).

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, an example of a manufacturing process of asemiconductor device is described with reference to FIGS. 16A and 16B,FIGS. 17A to 17F, FIGS. 18A to 18F, FIGS. 19A to 19D, FIGS. 20A to 20F,FIGS. 21A to 21D, FIGS. 22A-1 to 22B-2, and FIGS. 23A and 23C. Note thatmany portions of the semiconductor device and the manufacturing processthereof in this embodiment are the same as those in Embodiment 1.Therefore, description of the same portions is omitted and differentportions are described below in detail.

FIGS. 16A and 16B illustrate the semiconductor device of thisembodiment. FIG. 16A is a plan view, and FIG. 16B is a cross-sectionalview taken along line A-B in FIG. 16A.

Next, an example of a manufacturing process of the semiconductor deviceillustrated in FIGS. 16A and 16B is described with reference to FIGS.17A to 17F, FIGS. 18A to 18F, FIGS. 19A to 19D, FIGS. 20A to 20F, FIGS.21A to 21D, and FIGS. 22A-1 to 22B-2. Further, in this embodiment, thecase in which a semiconductor device is formed using a multi-tone maskis described.

First, a semiconductor layer 203 is formed over a substrate 200 havingan insulating surface (see FIGS. 17A and 17B).

As for the material of the substrate 200 and the material andmanufacturing method of the semiconductor layer 203, those of thesubstrate 100 and the semiconductor layer 103 a described in Embodiment1 can be referred to. In addition, an insulating film functioning as abase film may be formed over the substrate 200 having an insulatingsurface.

Next, a conductive film 204 and a conductive film 205 are formed overthe semiconductor layer 203 (see FIGS. 17C and 17D). As for thematerials and manufacturing methods of the conductive film 204 and theconductive film 205, those of the conductive film 104 and the conductivefilm 107 described in Embodiment 1 can be referred to.

Next, resist masks 206 a and 206 b are formed over the conductive film205. The resist masks 206 a and 206 b can be formed to have regions withdifferent thicknesses by using a multi-tone mask. By using themulti-tone mask, the number of photomasks used and the number ofmanufacturing steps can be reduced, which is preferable. In thisembodiment, the multi-tone mask can be used in a step of forming thepatterns of the conductive film 204 and the conductive film 205 and astep of forming the patterns of the conductive films 212 and 213 (seeFIGS. 19C and 19D).

A multi-tone mask is a mask capable of light exposure with multi-levellight intensity, typically, with three levels of light intensity so thatan exposed region, a semi-exposed region, and an unexposed region areformed. With the use of the multi-tone mask, a resist mask with pluralthicknesses (typically two kinds of thicknesses) can be formed byone-time exposure and development process. Therefore, with the use ofthe multi-tone mask, the number of photomasks can be reduced.

FIGS. 22A-1 and 22B-1 illustrate cross sections of typical multi-tonemasks. FIG. 22A-1 illustrates a gray-tone mask 403, and FIG. 22B-1illustrates a half-tone mask 414.

The gray-tone mask 403 illustrated in FIG. 22A-1 includes alight-blocking portion 401 formed using a light-blocking layer on alight-transmitting substrate 400, and a diffraction grating portion 402provided with a pattern of the light-blocking layer.

The diffraction grating portion 402 has slits, dots, meshes, or the likeprovided at intervals which are less than or equal to the resolutionlimit of light used for exposure, so that light transmittance iscontrolled. Note that the slits, dots, or meshes provided at thediffraction grating portion 402 may be provided periodically ornon-periodically.

For the light-transmitting substrate 400, quartz or the like can beused. The light-blocking layer included in the light-blocking portion401 and the diffraction grating portion 402 may be formed using a metalfilm, and is preferably formed using chromium, chromium oxide, or thelike.

In the case where the gray-tone mask 403 is irradiated with light forexposure, as illustrated in FIG. 22A-2, transmittance in a regionoverlapping with the light-blocking portion 401 is 0%, and transmittancein a region where neither the light-blocking portion 401 nor thediffraction grating portion 402 is provided is 100%. Further,transmittance at the diffraction grating portion 402 is approximately inthe range of 10 to 70%, which can be adjusted by the interval of slits,dots, or meshes of the diffraction grating, or the like.

The half-tone mask 414 illustrated in FIG. 22B-1 includes asemi-light-transmitting portion 412 formed using asemi-light-transmitting layer on a light-transmitting substrate 411, anda light-blocking portion 413 formed using a light-blocking layer.

The semi-light-transmitting portion 412 can be formed using a layer ofMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 413 may be formed using a metal film which is similar to that ofthe light-blocking layer of the gray-tone mask, and is preferably formedusing chromium, chromium oxide, or the like.

In the case where the half-tone mask 414 is irradiated with light forexposure, as illustrated in FIG. 22B-2, transmittance in a regionoverlapping with the light-blocking portion 413 is 0%, and transmittancein a region where neither the light-blocking portion 413 nor thesemi-light-transmitting portion 412 is provided is 100%. Further,transmittance at the semi-light-transmitting portion 412 isapproximately in the range of 10 to 70%, which can be adjusted by thekind, thickness, or the like of a material to be used.

Since a multi-tone photomask can achieve three levels of exposure toobtain an exposed portion, a half-exposed portion, and an unexposedportion, a resist mask with regions of a plurality of thicknesses(typically two kinds of thicknesses) can be formed by one-time exposureand development process. Thus, with the use of the multi-tone mask, thenumber of photomasks can be reduced.

A half-tone mask illustrated in FIGS. 17E and 17F includessemi-light-transmitting layers 301 a and 301 b and a light-blockinglayer 301 c on a light-transmitting substrate 300. Therefore, a resistmask provided in a portion serving as a source wiring later is formedthick and a resist mask provided in a portion serving as a sourceelectrode or a drain electrode later is formed thin over the conductivefilm 205 (see FIGS. 17E and 17F).

Unnecessary portions of the conductive films 204 and 205 are selectivelyetched away with the use of the resist masks 206 a and 206 b, so thatconductive layers 207 a and 208 a and the conductive layers 207 b and208 b are formed (see FIGS. 18A and 18B).

Next, ashing by oxygen plasma is performed on the resist masks 206 a and206 b. By performing ashing by oxygen plasma on the resist masks 206 aand 206 b, the resist mask 206 b is removed and the conductive layer 207b is exposed. In addition, the resist mask 206 a is reduced in size andremains as a resist mask 209 (see FIGS. 18C and 18D). By using theresist mask formed using the multi-tone mask in this manner, a resistmask is not additionally used, so that steps can be simplified.

Next, the conductive layers 207 a and 207 b are etched using the resistmask 209, so that a conductive layer 210 a is formed (see FIGS. 18E and18F). After the etching, the resist mask 209 is removed. Accordingly,the conductive layer 207 b is removed and the conductive layer 208 b isexposed. Further, part of the conductive layer 207 a, over which theresist mask 209 is not formed, is removed, so that the conductive layer208 a is exposed. The surface areas of the conductive layer 210 a whichis formed by the etching and the conductive layer 208 a are greatlydifferent from each other. That is, the surface area of the conductivelayer 208 a is larger than the surface area of the conductive layer 210a. Alternatively, as for the conductive layers 210 a and 208 a, thereare a region where the conductive layers 210 a and 208 a overlap witheach other and a region where the conductive layers 210 a and 208 a donot overlap with each other.

In the region where the conductive layers 208 a and 210 a overlap witheach other, the conductive layers 208 a and 210 a function as the sourcewiring. In the region where the conductive layers 208 a and 210 a do notoverlap with each other, the conductive layer 208 a functions as thesource electrode or the drain electrode. By forming the conductive layer208 a functioning as the source electrode or the drain electrode withthe use of a light-transmitting conductive material, the aperture ratioof the pixel can be improved. In addition, by forming the conductivelayer functioning as the source wiring by a stack of the conductivelayer 208 a and the conductive layer 210 a which has higher electricconductivity than the conductive layer 208 a, wiring resistance can bereduced, and power consumption can be reduced. Further, since the sourcewiring is formed using the light-blocking conductive layer 210 a, aspace between pixels can be shielded from light.

By using a multi-tone mask as described above, a light-transmittingregion (a region with high transmittance) and a light-blocking region (aregion with low transmittance) can be formed with one mask. Accordingly,the light-transmitting region (the region with high transmittance) andthe light-blocking region (the region with low transmittance) can beformed without an increase in the number of masks.

Next, after a gate insulating film 211 is formed over the conductivelayers 208 a and 208 b, conductive films 212 and 213 are formed over thegate insulating film 211 (see FIGS. 19A and 19B). As for the materialsand manufacturing methods of the conductive films 212 and 213, those ofthe gate insulating film 110, the conductive film 111, and theconductive film 114 described in Embodiment 1 can be referred to.

Next, resist masks 214 a and 214 b are formed over the conductive film213 with the use of a half-tone mask. The half-tone mask includessemi-light-transmitting layers 303 a and 303 b and light-blocking layers303 c and 303 d on the light-transmitting substrate 302. Therefore, aresist mask provided in a portion serving as a gate wiring later isformed thick and a resist mask provided in a portion serving as a gateelectrode later is formed thin over the conductive film 213 (see FIGS.19C and 19D).

Unnecessary portions of the conductive films 212 and 213 are selectivelyetched away with the use of the resist masks 214 a and 214 b, so thatconductive layers 215 a and 216 a and conductive layers 215 b and 216 bare formed (see FIGS. 20A and 20B).

Next, ashing by oxygen plasma is performed on the resist masks 214 a and214 b. By performing ashing by oxygen plasma on the resist masks 214 aand 214 b, the resist masks 214 a and 214 b are reduced in size andremain as resist masks 217 a and 217 b (see FIGS. 20C and 20D). By usingthe resist mask formed using the multi-tone mask in this manner, aresist mask is not additionally used, so that steps can be simplified.

Next, the conductive layers 215 a and 215 b are etched using the resistmasks 217 a and 217 b (see FIGS. 20E and 20F). Accordingly, parts of theconductive layers 215 a and 215 b, over which the resist masks 217 a and217 b are not formed, are removed, so that the conductive layers 216 aand 216 b are exposed. The surface areas of conductive layers 218 a and218 formed through the above step and the surface areas of theconductive layers 216 a and 216 b are greatly different from each other.That is, the surface areas of the conductive layers 216 a and 216 b arelarger than the surface areas of the conductive layers 218 a and 218 b.Alternatively, as for the conductive layers 216 a and 218 a, there are aregion where the conductive layers 216 a and 218 a overlap with eachother and a region where the conductive layers 216 a and 218 a do notoverlap with each other. Note that after the etching, the resist masks217 a and 217 b are removed.

A region including at least the conductive layer 218 a functions as thegate wiring, and a region including the conductive layer 216 a functionsas the gate electrode. By forming the conductive layer 216 a functioningas the gate electrode with the use of a light-transmitting conductivelayer, the aperture ratio of the pixel can be improved. In addition, byforming the conductive layer 216 a functioning as the gate wiring andthe conductive layer 218 a by a stack of the conductive layer 216 a andthe conductive layer 218 a which has higher electric conductivity thanthe conductive layer 216 a, wiring resistance can be reduced, and powerconsumption can be reduced. Further, since the gate wiring is formedusing the light-blocking conductive layer 218 a, the space between thepixels can be shielded from light. That is, with the gate wiringdisposed in a row direction and the source wiring disposed in a columndirection, the space between the pixels can be shielded from lightwithout use of a black matrix.

Further, a capacitor wiring is provided in the same direction as thegate wiring. The capacitor wiring is formed using the conductive layer216 b and the conductive layer 218 b which has higher electricconductivity than the conductive layer 216 b. By forming the capacitorwiring in this manner, wiring resistance can be lowered and powerconsumption can be reduced. Further, the conductive layer 216 bfunctions also as an electrode of a storage capacitor portion 260. Inthe storage capacitor, the storage capacitor portion 260 includes thegate insulating film 211 as a dielectric, and the conductive layers 208b and 216 b functioning as the electrodes.

By forming the storage capacitor portion 260 with the use of thelight-transmitting conductive layers as described above, light can betransmitted also in a portion where the storage capacitor portion 260 isformed. Thus, the aperture ratio can be improved. In addition, byforming the storage capacitor portion 260 with the use of thelight-transmitting conductive materials, the storage capacitor portion260 can be made larger. Thus, even when a transistor is turned off,potential holding properties of a pixel electrode is improved anddisplay quality is improved. Further, a feedthrough potential can belowered.

In this manner, a transistor 250 and the storage capacitor portion 260illustrated in FIGS. 16A and 16B can be formed.

Next, after an insulating film 219 is formed, a resist mask (notillustrated) is formed over the insulating film 219 and the insulatingfilm 219 is etched using the resist mask, so that a contact hole isformed in the insulating film 219 (see FIGS. 21A and 21B). Then, aconductive film 220 is formed over the insulating film 219 and thecontact hole. As for the materials and manufacturing methods of theinsulating film 219 and the conductive film 220, those of the insulatingfilm 117 and the conductive film 118 in Embodiment 1 can be referred to.Note that the insulating film 219 is not necessarily formed. The pixelelectrode may be formed over the same layer as the gate electrode andthe gate wiring.

Next, a resist mask (not illustrated) is formed over the conductive film220 and the conductive film 220 is selectively etched using the resistmask, so that conductive layers 221 a, 221 b, and 221 c are formed (seeFIGS. 21C and 21D). The conductive layers 221 a, 221 b, and 221 cfunction as pixel electrodes. Note that the resist mask is removed afterthe etching.

As described above, a semiconductor device can be manufactured. Since amulti-tone photomask can achieve three levels of exposure to obtain anexposed portion, a half-exposed portion, and an unexposed portion, aresist mask with regions of a plurality of thicknesses (typically twokinds of thicknesses) can be formed by one-time exposure and developmentprocess. Thus, with the use of the multi-tone mask, the number ofphotomasks can be reduced. By the manufacturing method described in thisembodiment, the light-transmitting transistor 250 and thelight-transmitting storage capacitor portion 260 can be formed.Therefore, since a wiring for connecting the transistor and an element(e.g., a different transistor) to each other can be formed using amaterial having low resistivity and high electric conductivity in apixel, waveform distortion of a signal can be suppressed and voltagedrop due to wiring resistance can be reduced.

Note that it is not necessary that light be transmitted through atransistor portion in a protection circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor in a pixel portion may be formed usinglight-transmitting materials and a transistor in the peripheral drivercircuit portion may be formed using a light-blocking material (see FIG.25B).

Although the case where a multi-tone mask is used for forming a sourcewiring, a source electrode, a gate wiring, and a gate electrode isdescribed in this embodiment, one embodiment of the present invention isnot limited to this. For example, a multi-tone mask can also be used forforming a semiconductor film, a source wiring, and a source electrode.Note that although the case where a multi-tone mask is used in both thestep of forming a gate wiring and the step of forming a source wiring isdescribed in this embodiment, the multi-tone mask may be used in eitherthe step of forming the gate wiring or the step of forming the sourcewiring. Further, the multi-tone mask can be used in the step of forminga semiconductor layer and the source wiring. FIG. 23A illustrates thecase where the semiconductor layer, the source wiring, and the sourceelectrode are formed using a multi-tone mask.

FIG. 23B illustrates the case where the semiconductor layer, the sourcewiring, and the source electrode are formed using a multi-tone mask anda storage capacitor portion is formed. In addition, a multi-tone maskcan also be used in the case where a channel protective film is formedover a channel formation region of a semiconductor film (see FIG. 23C).Since a semiconductor layer of the transistor 250 and an oxidesemiconductor layer of the storage capacitor portion 260 are formed inone island in FIGS. 23B and 23C, layout for forming the oxidesemiconductor layer can be facilitated. Further, since the number ofcontact holes can be reduced, contact resistance can be lowered.Further, contact defects can be reduced.

Next, FIG. 35A illustrates the case where a semiconductor layer 203 band a conductive layer 210 a functioning as a source wiring are formedusing a multi-tone mask. Further, FIG. 35B illustrates the case wherethe semiconductor layer 203 b and conductive layers 208 c and 208 dfunctioning as a source electrode and a drain electrode are formed usinga multi-tone mask.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, an example in which at least part of a drivercircuit and a thin film transistor provided in a pixel portion areformed over the same substrate in a display device is described.

FIG. 24A is an example of a block diagram of an active matrix liquidcrystal display device which is an example of a display device. Thedisplay device illustrated in FIG. 24A includes, over a substrate 5300,a pixel portion 5301 including a plurality of pixels each having adisplay element; a scan line driver circuit 5302 for selecting a pixel;and a signal line driver circuit 5303 for controlling a video signalwhich is input to the selected pixel.

A light-emitting display device illustrated in FIG. 24B includes, over asubstrate 5400, a pixel portion 5401 including a plurality of pixelseach having a display element; a first scan line driver circuit 5402 anda second scan line driver circuit 5404 for selecting a pixel; and asignal line driver circuit 5403 for controlling a video signal which isinput to the selected pixel.

In the case where a video signal which is input to a pixel of thelight-emitting display device illustrated in FIG. 24B is a digitalsignal, the pixel is in a light-emitting state or in anon-light-emitting state by switching of on/off of a transistor. Thus,gray levels can be displayed using an area ratio gray scale method or atime ratio gray scale method. An area ratio gray scale method refers toa driving method by which one pixel is divided into a plurality ofsubpixels and the subpixels are driven separately on the basis of videosignals so that gray levels are displayed. Further, a time ratio grayscale method refers to a driving method by which a period during whichlight emitted in a pixel is controlled so that gray levels aredisplayed.

Since the response time of a light-emitting element is shorter than theresponse time of a liquid crystal element or the like, thelight-emitting element is more suitable for a time ratio gray scalemethod than the liquid crystal element. In the case of performingdisplay by a time ratio gray scale method, one frame period is dividedinto a plurality of subframe periods. Then, in accordance with videosignals, the light-emitting element in the pixel is set to be in alight-emitting state or a non-light-emitting state in each subframeperiod. By dividing one frame period into a plurality of subframesperiods, the total length of time during which light is emitted inpixels in one frame period can be controlled with video signals, so thatgray levels can be displayed.

Note that in the light-emitting display device illustrated in FIG. 24B,in the case where two switching TFTs are provided in one pixel, thefirst scan line driver circuit 5402 generates a signal which is input toa first scan line serving as a gate wiring of one of the switching TFTs,and the second scan line driver circuit 5404 generates a signal which isinput to a second scan line serving as a gate wiring of the other of theswitching TFTs; however, one scan line driver circuit may generate boththe signal which is input to the first scan line and the signal which isinput to the second scan line. In addition, for example, there is apossibility that a plurality of scan lines used for controlling theoperation of the switching element are provided in each pixel, dependingon the number of the switching TFTs included in one pixel. In such acase, one scan line driver circuit may generate all signals that areinput to the plurality of scan lines, or a plurality of scan line drivercircuits may generate all the signals that are input to the plurality ofscan lines.

The thin film transistor to be provided in the pixel portion of theliquid crystal display device is formed as in Embodiment 1 or 2.Further, since the thin film transistors described in Embodiments 1 and2 are n-channel TFTs, part of a driver circuit which can be formed usingan n-channel TFT among driver circuits is formed over the same substrateas the thin film transistor in the pixel portion.

Also in the light-emitting display device, part of a driver circuitwhich can be formed using an n-channel TFT among driver circuits can beformed over the same substrate as the thin film transistor in the pixelportion. Alternatively, the signal line driver circuit and the scan linedriver circuit can be formed using only the n-channel TFTs described inEmbodiments 1 and 2.

Note that it is not necessary that light be transmitted through atransistor in a protection circuit or a peripheral driver circuitportion such as a gate driver or a source driver. Thus, light istransmitted through a transistor and a capacitor in a pixel portion, andlight is not necessarily transmitted through a transistor in theperipheral driver circuit portion.

FIG. 25A illustrates the case where a thin film transistor is formedwithout use of a multi-tone mask, and FIG. 25B illustrates the casewhere a thin film transistor is formed using a multi-tone mask. The thinfilm transistor formed without use of a multi-tone mask includes asemiconductor layer 171 provided over the substrate 100 having aninsulating surface, conductive layers 172 which are provided over thesemiconductor layer 171 and function as a source electrode and a drainelectrode, the gate insulating film 110 provided over the conductivelayers 172, and a conductive layer 174 which is provided over the gateinsulating film 110 and functions as a gate electrode. The conductivelayer 174 functioning as the gate electrode and the conductive layers172 functioning as the source electrode and the drain electrode can beformed using light-blocking conductive layers (see FIG. 25A). Further,an insulating film 175 is formed over the conductive layer 174functioning as the gate electrode.

The thin film transistor formed using a multi-tone mask includes asemiconductor layer 271 provided over the substrate 200 having aninsulating surface, conductive layers 272 and 273 which are providedover the semiconductor layer 271 and function as a source electrode anda drain electrode, a gate insulating film provided over the conductivelayers 273, and conductive layers 275 and 276 which are provided overthe gate insulating film and function as a gate electrode. The gateelectrode, the source electrode, and the drain electrode can be eachformed by stacking a light-transmitting conductive layer and alight-blocking conductive layer (see FIG. 25B). Further, an insulatingfilm 277 is formed over the conductive layers 275 and 276 functioning asthe gate electrode.

Note that it is not necessary that light be transmitted through atransistor in a protection circuit or a peripheral driver circuitportion such as a gate driver or a source driver. Therefore, for asemiconductor layer used in one embodiment of the present invention, aswell as an oxide semiconductor, any of a crystalline semiconductor (asingle crystal semiconductor or a polycrystalline semiconductor), anamorphous semiconductor, a microcrystalline semiconductor, an organicsemiconductor, and the like may be used.

Further, the above driver circuit may be used in an electronic paper inwhich electronic ink is driven using an element which is electricallyconnected to a switching element, without limitation to a liquid crystaldisplay device or a light-emitting display device. An example ofelectronic paper is an electrophoretic display device (electrophoreticdisplay) or the like. Electronic paper has advantages of the same levelof readability as plain paper, lower power consumption than otherdisplay devices, and reduction in thickness and weight.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 4

Next, the structure of a display device which is one embodiment of asemiconductor device is described. In this embodiment, a light-emittingdisplay device including a light-emitting element utilizingelectroluminescence is described as a display device. Light-emittingelements utilizing electroluminescence are classified according towhether a light-emitting material is an organic compound or an inorganiccompound. In general, the former is referred to as an organic ELelement, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are injected from a pair of electrodes intoa layer containing a light-emitting organic compound, and current flows.These carriers (electrons and holes) are recombined, so that thelight-emitting organic compound is excited. The light-emitting organiccompound emits light in returning to a ground state from the excitedstate. Due to such a mechanism, such a light-emitting element isreferred to as a current-excitation light-emitting element.

Inorganic EL elements are classified according to their elementstructures into dispersion-type inorganic EL elements and thin-filminorganic EL elements. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission which utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is interposed between dielectriclayers, which are further interposed between electrodes, and its lightemission mechanism is localized type light emission which utilizesinner-shell electron transition of metal ions. Note that an organic ELelement is used as a light-emitting element in this example.

Next, a structure and operation of a pixel to which digital time ratiogray scale driving can be applied is described. FIGS. 26A and 26Billustrate examples of pixel structures to which digital time gray scaledriving can be applied. Here, an example is described in which one pixelincludes two n-channel transistors each having a semiconductor layer asa channel formation region.

A pixel 6400 illustrated in FIG. 26A includes a switching transistor6401, a driving transistor 6402, a light-emitting element 6404, and acapacitor 6403. A gate of the switching transistor 6401 is connected toa scan line 6406. A first electrode (one of a source electrode and adrain electrode) of the switching transistor 6401 is connected to asignal line 6405. A second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 6401 is connectedto a gate of the driving transistor 6402. The gate of the drivingtransistor 6402 is connected to a power supply line 6407 through thecapacitor 6403. A first electrode of the driving transistor 6402 isconnected to the power supply line 6407. A second electrode of thedriving transistor 6402 is connected to a first electrode (a pixelelectrode) of the light-emitting element 6404. A second electrode of thelight-emitting element 6404 corresponds to a common electrode 6408.

Note that the second electrode (the common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Notethat the low power supply potential is a potential satisfying the lowpower supply potential<a high power supply potential with reference tothe high power supply potential which is set to the power supply line6407. As the low power supply potential, GND, 0 V, or the like may beemployed, for example. A potential difference between the high powersupply potential and the low power supply potential is applied to thelight-emitting element 6404 and current flows to the light-emittingelement 6404, so that the light-emitting element 6404 emits light. Here,in order to make the light-emitting element 6404 emit light, eachpotential is set so that the potential difference between the high powersupply potential and the low power supply potential is forward thresholdvoltage (Vth) or higher of the light-emitting element 6404.

Note gate capacitance of the driving transistor 6402 may be used as asubstitute for the capacitor 6403, so that the capacitor 6403 can beeliminated. The gate capacitance of the driving transistor 6402 may beformed between a channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate of the driving transistor 6402 so that the drivingtransistor 6402 is in either of two states of being sufficiently turnedon or turned off. That is, the driving transistor 6402 operates in alinear region. Since the driving transistor 6402 operates in the linearregion, voltage which is higher than the voltage of the power supplyline 6407 is applied to the gate of the driving transistor 6402. Notethat voltage which is higher than or equal to (voltage of the powersupply line +Vth of the driving transistor 6402) is applied to thesignal line 6405.

In the case of using an analog gray scale method instead of the digitaltime ratio gray scale method, the same pixel structure as in FIG. 26Acan be used by changing signal input.

In the case of performing analog gray scale driving, voltage which ishigher than or equal to (forward voltage of the light-emitting element6404 and Vth of the driving transistor 6402) is applied to the gate ofthe driving transistor 6402. The forward voltage of the light-emittingelement 6404 refers to voltage at which desired luminance is obtainedand refers to at least forward threshold voltage. Note that a videosignal by which the driving transistor 6402 operates in a saturationregion is input, so that current can flow to the light-emitting element6404. In order for the driving transistor 6402 to operate in thesaturation region, a potential of the power supply line 6407 is set to apotential which is higher than a gate potential of the drivingtransistor 6402. When an analog video signal is used as a video signal,current corresponding to the video signal can flow to the light-emittingelement 6404, and the analog gray scale driving can be performed.

Note that the pixel structure is not limited to the pixel structure inFIG. 26A. For example, the pixel in FIG. 26A can further include aswitch, a resistor, a capacitor, a transistor, a logic circuit, or thelike. For example, a pixel structure illustrated in FIG. 26B may beused. A pixel 6410 illustrated in FIG. 26B includes the switchingtransistor 6401, the driving transistor 6402, the light-emitting element6404, and the capacitor 6403. The gate of the switching transistor 6401is connected to the scan line 6406. The first electrode (the one of thesource electrode and the drain electrode) of the switching transistor6401 is connected to the signal line 6405. The second electrode (theother of the source electrode and the drain electrode) of the switchingtransistor 6401 is connected to the gate of the driving transistor 6402.The gate of the driving transistor 6402 is connected to the firstelectrode (the pixel electrode) of the light-emitting element 6404through the capacitor 6403. The first electrode of the drivingtransistor 6402 is connected to the wiring 6426 for applying pulsevoltage. The second electrode of the driving transistor 6402 isconnected to the first electrode of the light-emitting element 6404. Thesecond electrode of the light-emitting element 6404 corresponds to thecommon electrode 6408. Needless to say, a switch, a resistor, acapacitor, a transistor, a logic circuit, or the like may be added tothis structure.

Next, structures of the light-emitting element are described withreference to FIGS. 27A to 27C. Here, cross-sectional structures ofpixels are described by taking the case where the transistor 150illustrated in FIGS. 10A and 10B is used as a driving TFT as an example.Driving TFTs 7001, 7011, and 7021 used in semiconductor devicesillustrated in FIGS. 27A to 27C can be manufactured in a manner similarto those of the thin film transistors described in Embodiments 1 and 2and are thin film transistors with favorable electrical characteristicseach having an oxide semiconductor as a semiconductor layer.

In order to extract light emitted from the light-emitting element, atleast one of an anode and a cathode may be transparent. A thin filmtransistor and a light-emitting element are formed over a substrate. Alight-emitting element can have a top emission structure in which lightis extracted through a surface which is opposite to the substrate; abottom emission structure in which light is extracted through a surfaceon the substrate side; or a dual emission structure in which light isextracted through a surface which is opposite to the substrate and asurface on the substrate side. The pixel structures illustrated in FIGS.26A and 26B can be applied to a light-emitting element having any ofthese emission structures.

A light-emitting element having a top emission structure is describedwith reference to FIG. 27A.

FIG. 27A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 corresponds to the transistor 150 illustrated in FIGS.10A and 10B and light is emitted from a light-emitting element 7002 tothe anode 7005 side. In FIG. 27A, a cathode 7003 of the light-emittingelement 7002 and the driving TFT 7001 are electrically connected to eachother, and a light-emitting layer 7004 and the anode 7005 are stacked inthat order over the cathode 7003. The cathode 7003 can be formed using avariety of conductive materials as long as they have low work functionsand reflect light. For example, Ca, Al, MgAg, AlLi, or the like ispreferably used. The light-emitting layer 7004 may be formed usingeither a single layer or a plurality of layers stacked. In the casewhere the light-emitting layer 7004 is formed using a plurality oflayers, the light-emitting layer 7004 is formed by stacking an electroninjection layer, an electron transport layer, a light-emitting layer, ahole transport layer, and a hole injection layer in that order over thecathode 7003. Note that it is not necessary to provide all these layers.For example, the anode 7005 is formed using a light-transmittingconductive material such as a film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thelight-emitting layer 7004 is interposed between the cathode 7003 and theanode 7005. In the case of the pixel illustrated in FIG. 27A, light isemitted from the light-emitting element 7002 to the anode 7005 side, asindicated by an arrow.

Note that the gate electrode provided over the semiconductor layer inthe driver circuit is preferably formed using the same material as thecathode 7003 because the process can be simplified. An insulating filmmay be formed over the anode. For example, since SiN_(x) and SiO_(x)have hygroscopic properties, they can prevent the EL element fromdeteriorating. Further, when the cathode is formed using a transflectivefilm (having a transmittance of 30 to 80% and a reflectivity of 30 to60%) and a micro-cavity structure (a micro resonator) is applied, colorpurity can be improved.

Next, a light-emitting element having a bottom emission structure isdescribed with reference to FIG. 27B. FIG. 27B is a cross-sectional viewof a pixel in the case where the driving TFT 7011 corresponds to thetransistor 150 illustrated in FIGS. 10A and 10B and light is emittedfrom a light-emitting element 7012 to the cathode 7013 side. In FIG.27B, the cathode 7013 of the light-emitting element 7012 is formed overa light-transmitting conductive film 7017 which is electricallyconnected to the driving TFT 7011, and a light-emitting layer 7014 andan anode 7015 are stacked in that order over the cathode 7013. Note thata light-blocking film 7016 for reflecting or blocking light may beformed so as to cover the anode 7015 in the case where the anode 7015has light-transmitting properties. As in FIG. 27A, the cathode 7013 canbe formed using a variety of conductive materials as long as they havelow work functions. Note that the cathode 7013 is formed to a thicknessthat allows light transmission (preferably, approximately 5 to 30 nm).For example, a 20-nm-thick aluminum film can be used as the cathode7013. As in FIG. 27A, the light-emitting layer 7014 may be formed usingeither a single layer or a plurality of layers stacked. The anode 7015does not need to transmit light, but can be formed using alight-transmitting conductive material, as in FIG. 27A. For thelight-blocking film 7016, metal or the like which reflects light can beused, for example; however, the light-blocking film 7016 is not limitedto a metal film. For example, a resin to which a black pigment is addedor the like can be used.

The light-emitting element 7012 corresponds to a region where thelight-emitting layer 7014 is interposed between the cathode 7013 and theanode 7015. In the case of the pixel illustrated in FIG. 27B, light isemitted from the light-emitting element 7012 to the cathode 7013 side,as indicated by an arrow.

Note that the gate electrode provided over the semiconductor layer inthe driver circuit is preferably formed using the same material as thecathode 7013 because the process can be simplified.

Next, a light-emitting element having a dual emission structure isdescribed with reference to FIG. 27C. In FIG. 27C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive layer 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthat order over the cathode 7023. As in FIG. 27A, the cathode 7023 canbe formed using a variety of conductive materials as long as they havelow work functions. Note that the cathode 7023 is formed to a thicknessthat allows light transmission. For example, 20-nm-thick Al can be usedfor the cathode 7023. As in FIG. 27A, the light-emitting layer 7024 maybe formed using either a single layer or a plurality of layers stacked.The anode 7025 can be formed using a light-transmitting conductivematerial, as in FIG. 27A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 27C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side, as indicated by arrows.

Note that the gate electrode provided over the semiconductor layer inthe driver circuit is preferably formed using the same material as theconductive layer 7027 because the process can be simplified. Further,the gate electrode provided over the semiconductor layer in the drivercircuit is preferably formed by stacking the material used for theconductive layer 7027 and the material used for the cathode 7023 becausethe process can be simplified and wiring resistance can be lowered.

Note that although the organic EL elements are described here as thelight-emitting elements, an inorganic EL element can be provided as alight-emitting element. An anode may be used in common among all thepixels and a cathode may be patterned into a pixel electrode.

Note that in this embodiment, an example is described in which a thinfilm transistor (a driving TFT) which controls driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT forcontrolling current is connected between the driving TFT and thelight-emitting element.

Note that the structure of a semiconductor device described in thisembodiment is not limited to the structures illustrated in FIGS. 27A to27C and can be modified in various ways on the basis of the spirit oftechniques disclosed.

Next, a top surface and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which is oneembodiment of a semiconductor device, are described with reference toFIGS. 28A and 28B. FIG. 28A is a top view of a panel in which a thinfilm transistor and a light-emitting element which are formed over afirst substrate are sealed between the first substrate and a secondsubstrate with a sealant. FIG. 28B is a cross-sectional view taken alongline H-I in FIG. 28A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Thus, the pixel portion4502, the signal line driver circuits 4503 a and 4503 b, and the scanline driver circuits 4504 a and 4504 b are sealed together with a filler4507, by the first substrate 4501, the sealant 4505, and the secondsubstrate 4506. It is preferable that the panel be packaged (sealed)with a protective film (e.g., an attachment film or an ultravioletcurable resin film) or a cover material, which has high air-tightnessand causes less degasification, so that the panel is not exposed to theexternal air, in this manner.

Further, the pixel portion 4502, the signal line driver circuits 4503 aand 4503 b, and the scan line driver circuits 4504 a and 4504 b whichare provided over the first substrate 4501 each include a plurality ofthin film transistors, and a thin film transistor 4510 included in thepixel portion 4502 and a thin film transistor 4509 included in thesignal line driver circuit 4503 a are illustrated as examples in FIG.28B. For each of the thin film transistors 4509 and 4510, a highlyreliable thin film transistor including an oxide semiconductor as itssemiconductor layer as described in Embodiment 1 or 2 can be applied.

Note that it is not necessary that light be transmitted through atransistor portion in a protection circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor in the pixel portion 4502 may be formed usinglight-transmitting materials and a transistor in the peripheral drivercircuit portion may be formed using a light-blocking material.

Further, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode of thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that although the light-emitting element 4511 has a layeredstructure of the first electrode layer 4517, an electroluminescent layer4512, and a second electrode layer 4513, the structure of thelight-emitting element 4511 is not limited to the structure described inthis embodiment. The structure of the light-emitting element 4511 can bechanged as appropriate depending on a direction in which light isextracted from the light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material and anopening be formed over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition 4520 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b,the scan line driver circuits 4504 a and 4504 b, or the pixel portion4502.

A connection terminal electrode 4515 may be formed using the sameconductive film as the first electrode layer 4517 of the light-emittingelement 4511, and a terminal electrode 4516 may be formed using the sameconductive film as the source electrode layers and the drain electrodelayers of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to havelight-transmitting properties. In this case, a light-transmittingmaterial such as a glass plate, a plastic plate, a polyester film, or anacrylic film is used.

Further, as well as an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used as thefiller 4507. PVC (polyvinyl chloride), acrylic, polyimide, an epoxyresin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylene vinylacetate) can be used.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on a surface so that glare canbe reduced.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be provided by mounting drivercircuits formed using a single crystal semiconductor substrate or asingle crystal semiconductor film or a polycrystalline semiconductorfilm over an insulating substrate separately prepared. Alternatively,only the signal line driver circuits or part thereof, or only the scanline driver circuits or part thereof may be separately formed andmounted. This embodiment is not limited to the structure illustrated inFIGS. 28A and 28B.

Through the above process, a light-emitting display device can bemanufactured at lower manufacturing cost.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 5

Next, a different structure of a display device which is one embodimentof a semiconductor device is described. In this embodiment, a liquidcrystal display device including a liquid crystal element is describedas a display device.

First, a top surface and a cross section of a liquid crystal displaypanel (also referred to as a liquid crystal panel) which is oneembodiment of a liquid crystal display device are described withreference to FIGS. 29A-1, 29A-2, and 29B. FIGS. 29A-1 and 29A-2 are topviews of a panel in which highly reliable thin film transistors 4010 and4011 each including the oxide semiconductor as its semiconductor layeras described in Embodiment 1 or 2 and a liquid crystal element 4013,which are formed over a first substrate 4001, are sealed between thefirst substrate 4001 and a second substrate 4006 with a sealant 4005.FIG. 29B is a cross-sectional view taken along line M-N in FIGS. 29A-1and 29A-2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. In addition, the second substrate 4006 is provided overthe pixel portion 4002 and the scan line driver circuit 4004. Thus, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. Further, a signal linedriver circuit 4003 which is formed using a single crystal semiconductorfilm or a polycrystalline semiconductor film over a substrate separatelyprepared is mounted on a region which is different from a regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that the connection method of a driver circuit which is separatelyformed is not particularly limited to a certain method, and a COGmethod, a wire bonding method, a TAB method, or the like can be used.FIG. 29A-1 illustrates an example of mounting the signal line drivercircuit 4003 by a COG method, and FIG. 29A-2 illustrates an example ofmounting the signal line driver circuit 4003 by a TAB method.

Further, the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 each include aplurality of thin film transistors, and the thin film transistor 4010included in the pixel portion 4002 and the thin film transistor 4011included in the scan line driver circuit 4004 are illustrated asexamples in FIG. 29B. Over the thin film transistors 4010 and 4011, aninsulating layer 4021 is provided. For each of the thin film transistors4010 and 4011, a highly reliable thin film transistor including an oxidesemiconductor as its semiconductor layer as described in Embodiment 1 or2 can be applied.

Note that it is not necessary that light be transmitted through atransistor portion in a protection circuit or a peripheral drivercircuit portion such as a gate driver or a source driver. Thus, atransistor and a capacitor in the pixel portion 4002 may be formed usinglight-transmitting materials and a transistor in the peripheral drivercircuit portion may be formed using a light-blocking material.

In addition, a pixel electrode 4030 of the liquid crystal element 4013is electrically connected to the thin film transistor 4010. Further, acounter electrode layer 4031 of the liquid crystal element 4013 isprovided on the second substrate 4006. A portion where the pixelelectrode 4030, the counter electrode layer 4031, and the liquid crystallayer 4008 overlap with one another corresponds to the liquid crystalelement 4013. Note that the pixel electrode 4030 and the counterelectrode layer 4031 are provided with an insulating layer 4032 and aninsulating layer 4033 each functioning as an alignment film, and holdthe liquid crystal layer 4008 with the insulating layers 4032 and 4033interposed therebetween.

In the pixel portion 4002 except a lattice-like wiring portion, lightcan be transmitted, so that the aperture ratio can be improved. Further,a space is needed between pixel electrodes and an electric field is notapplied to a liquid crystal in the space portion. Therefore, it ispreferable that light be not transmitted in the space portion. Thus, thelattice-like wiring portion can be used as a black matrix.

Note that the first substrate 4001 and the second substrate 4006 can beformed using glass, metal (typically stainless steel), ceramics, orplastics. As plastics, a fiberglass-reinforced plastic (FRP) plate, apolyvinyl fluoride (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in whichaluminum foil is interposed between PVF films or polyester films can beused.

In addition, reference numeral 4035 denotes a columnar spacer obtainedby selective etching of an insulating film and is provided forcontrolling the distance between the pixel electrode 4030 and thecounter electrode layer 4031 (a cell gap). Note that a spherical spacermay be used. Further, the counter electrode layer 4031 is electricallyconnected to a common potential line provided over the same substrate asthe thin film transistor 4010. With the use of a common connectionportion, the counter electrode layer 4031 and the common potential linecan be electrically connected to each other by conductive particlesdisposed between the pair of substrates. Note that the conductiveparticles are contained in the sealant 4005.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is not used may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of a cholestericliquid crystal is increased. Since the blue phase is generated within anonly narrow range of temperature, a liquid crystal compositioncontaining a chiral agent at 5 weight percent or more in order toimprove the temperature range is used for the liquid crystal layer 4008.The liquid crystal composition which includes a liquid crystalexhibiting a blue phase and a chiral agent has a short response time of10 to 100 μs, has optical isotropy, which makes alignment treatmentunneeded, and has small viewing angle dependency.

Although the liquid crystal display device described in this embodimentis an example of a transmissive liquid crystal display device, theliquid crystal display device described in this embodiment can beapplied to either a reflective liquid crystal display device or atransflective liquid crystal display device.

The liquid crystal display device described in this embodiment is anexample in which a polarizing plate is provided on the outer surface ofthe substrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are provided on the inner surface ofthe substrate in that order; however, the polarizing plate may beprovided on the inner surface of the substrate. In addition, the layeredstructure of the polarizing plate and the coloring layer is not limitedto the layered structure in this embodiment and may be set asappropriate depending on materials of the polarizing plate and thecoloring layer or conditions of the manufacturing process. Further, alight-blocking film functioning as a black matrix may be provided.

In this embodiment, in order to reduce surface unevenness of the thinfilm transistor and to improve reliability of the thin film transistor,the thin film transistor obtained in Embodiment 1 or 2 is covered withthe insulating layer 4021 functioning as a protective film or aplanarizing insulating film. The insulating layer 4021 can be formed tohave a single-layer structure or a layered structure of two or morelayers. Note that the protective film is provided in order to prevententry of contaminant impurities such as organic substance, metal, ormoisture existing in the air and is preferably a dense film. Theprotective film may be formed to have a single layer or a stacked layerof a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm, a silicon nitride oxide film, an aluminum oxide film, an aluminumnitride film, an aluminum oxynitride film, and/or an aluminum nitrideoxide film by sputtering. Although this embodiment describes an exampleof forming the protective film by sputtering, this embodiment is notparticularly limited to this method and any of a variety of methods suchas plasma-enhanced CVD may be used.

An insulating layer having a layered structure can be formed for theprotective film. In the case of forming an insulating layer having alayered structure, as a first layer of the protective film, for example,a silicon oxide film is formed by sputtering. When the silicon oxidefilm is used as the protective film, the silicon oxide film has aneffect of preventing hillocks of an aluminum film used for the sourceelectrode layer and the drain electrode layer.

Further, as a second layer of the protective film, for example, asilicon nitride film is formed by sputtering. When the silicon nitridefilm is used as the protective film, mobile ions of sodium or the likecan be prevented from entering a semiconductor region so that theelectrical characteristics of the TFT are not changed.

In addition, after the protective film is formed, the semiconductorlayer may be subjected to annealing (300 to 400° C.).

The insulating layer 4021 is formed as the planarizing insulating film.For the insulating layer 4021, an organic material having heatresistance, such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy can be used. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formedusing these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent. Inaddition, the organic group may include a fluoro group.

The formation method of the insulating layer 4021 is not particularlylimited to a certain method, and any of the following methods can beemployed depending on the material: sputtering, an SOG method, spincoating, dipping, spray coating, a droplet discharge method (e.g., anink jet method, screen printing, or offset printing), a doctor knife, aroll coater, a curtain coater, a knife coater, and the like. In the casewhere the insulating layer 4021 is formed using a material solution, thesemiconductor layer may be annealed (at 300 to 400° C.) at the same timeas a baking step of the insulating layer 4021. The baking step of theinsulating layer 4021 also serves as annealing of the semiconductorlayer, so that a semiconductor device can be manufactured efficiently.

The pixel electrode 4030 and the counter electrode layer 4031 can beformed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Alternatively, the pixel electrode 4030 and the counter electrode layer4031 can be formed using a conductive composition containing aconductive high molecule (also referred to as a conductive polymer). Thepixel electrode formed using the conductive composition preferably has asheet resistance less than or equal to 10000 ohms/square and atransmittance greater than or equal to 70% at a wavelength of 550 nm.The sheet resistance of the pixel electrode is preferably lower.Further, the resistivity of the conductive high molecule contained inthe conductive composition is preferably 0.1 ohm·cm or less.

As the conductive high molecule, a so-called π electron conjugatedconductive high molecule can be used. For example, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more kinds of them, orthe like can be used.

Further, a variety of signals and potentials are supplied from an FPC4018 to the signal line driver circuit 4003 which is formed separately,the scan line driver circuit 4004, or the pixel portion 4002.

A connection terminal electrode 4015 may be formed using the sameconductive film as the pixel electrode 4030 of the liquid crystalelement 4013, and a terminal electrode 4016 may be formed using the sameconductive film as the source electrode layers and the drain electrodelayers of the thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Although FIGS. 29A-1 and 29A-2 illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, this embodiment is not limited to this structure. Thescan line driver circuit may be formed separately and mounted, or onlypart of the signal line driver circuit or part of the scan line drivercircuit may be formed separately and mounted.

FIG. 30 illustrates an example in which a liquid crystal display moduleis formed as a semiconductor device by using a TFT substrate 2600.

FIG. 30 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer, acoloring layer 2605, and a polarizing plate 2606 are provided betweenthe substrates to form a display region. The coloring layer 2605 isnecessary to perform color display. In the RGB system, coloring layerscorresponding to colors of red, green, and blue are provided for pixels.Polarizing plates 2606 and 2607 and a diffusion plate 2613 are providedoutside the TFT substrate 2600 and the counter substrate 2601. A lightsource includes a cold cathode fluorescent lamp 2610 and a reflector2611, and a circuit board 2612 is connected to a wiring circuit portion2608 of the TFT substrate 2600 by a flexible wiring board 2609 andincludes an external circuit such as a control circuit or a power supplycircuit. The polarizing plate and the liquid crystal layer may bestacked with a retardation plate interposed therebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned microcell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (anti-ferroelectric liquid crystal) mode,or the like can be used.

Through the above process, a liquid crystal display device can bemanufactured at lower manufacturing cost.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 6

Next, electronic paper which is one embodiment of a semiconductor deviceis described. Electronic paper has advantages of the same level ofreadability as plain paper, lower power consumption than other displaydevices, and reduction in thickness and weight.

FIG. 31 illustrates active matrix electronic paper as one embodiment ofa semiconductor device. A thin film transistor 581 used for a pixelportion of the semiconductor device can be formed in a manner similar tothat of the thin film transistor in the pixel portion described in theabove embodiment and is a thin film transistor including an oxidesemiconductor as a semiconductor layer.

The electronic paper in FIG. 31 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control the orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 formed over a substrate 580 is a top-gatethin film transistor in which a source electrode layer and a drainelectrode layer are in contact with and electrically connected to afirst electrode layer 587 through an opening formed in an insulatinglayer 585. Between the first electrode layer 587 and a second electrodelayer 588 provided for a substrate 586, spherical particles 589 eachhaving a black region 590 a, a white region 590 b, and a cavity 594filled with liquid around the black region 590 a and the white region590 b are provided. A space around the spherical particles 589 is filledwith a filler 595 such as a resin (see FIG. 31).

Alternatively, instead of the twisting ball, an electrophoretic elementcan be used. A microcapsule having a diameter of approximately 10 to 200μm, in which transparent liquid, white microparticles which are chargedpositively or negatively, and black microparticles which are charged topolarity different from that of the white microparticles areencapsulated, is used. In the microcapsule which is provided between thefirst electrode layer and the second electrode layer, when an electricfield is applied by the first electrode layer and the second electrodelayer, the white microparticles and the black microparticles move toopposite sides, so that white or black can be displayed. A displayelement using this principle is an electrophoretic display element.Since the electrophoretic display element has higher reflectivity than aliquid crystal element, an auxiliary light is not needed, powerconsumption is low, and a display portion can be recognized in a dimplace. In addition, even when power is not supplied to the displayportion, an image which has been displayed once can be maintained.Accordingly, a displayed image can be maintained even when electronicpaper is distanced from a power supply source (e.g., a source of radiowaves).

Through the above process, electronic paper can be manufactured at lowermanufacturing cost.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 7

A semiconductor device of this embodiment can be used in a variety ofelectronic devices (including an amusement machine). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, a large gamemachine such as a pinball machine, and the like.

FIG. 32A illustrates an example of a portable information terminaldevice 9200. The portable information terminal device 9200 incorporatesa computer and can perform a variety of data processings. An example ofthe portable information terminal device 9200 is a personal digitalassistant (PDA).

The portable information terminal device 9200 includes two housings: ahousing 9201 and a housing 9203. The housing 9201 and the housing 9203are joined to each other with a joining portion 9207 so that theportable information terminal device 9200 can be foldable. A displayportion 9202 is incorporated in the housing 9201, and the housing 9203is provided with a keyboard 9205. Needless to say, the structure of theportable information terminal device 9200 is not limited to the abovestructure, and the structure may include at least the thin filmtransistor described in Embodiment 1 or 2, and an additional accessorycan be provided as appropriate. A driver circuit and a pixel portion areformed over the same substrate, which leads to reduction inmanufacturing cost. Thus, a portable information terminal device havinga thin film transistor with favorable electrical characteristics can berealized.

FIG. 32B illustrates an example of a digital video camera 9500. Thedigital video camera 9500 includes a display portion 9503 incorporatedin a housing 9501 and a variety of operation portions. Note that thestructure of the digital video camera 9500 is not limited to a certainstructure, and the structure may include at least the thin filmtransistor described in Embodiment 1 or 2, and an additional accessorycan be provided as appropriate. A driver circuit and a pixel portion areformed over the same substrate, which leads to reduction inmanufacturing cost. Thus, a digital video camera having a thin filmtransistor with favorable electrical characteristics can be realized.

FIG. 32C illustrates an example of a mobile phone 9100. The mobile phone9100 has two housings: a housing 9104 and a housing 9101. The housing9104 and the housing 9101 are joined to each other with a joiningportion 9103 so that the mobile phone can be foldable. A display portion9102 is incorporated in the housing 9104, and the housing 9101 isprovided with operation keys 9106. Note that the structure of the mobilephone 9100 is not limited to a certain structure, and the structure mayinclude at least the thin film transistor described in Embodiment 1 or2, and an additional accessory can be provided as appropriate. A drivercircuit and a pixel portion are formed over the same substrate, whichleads to reduction in manufacturing cost. Thus, a mobile phone having athin film transistor with favorable electrical characteristics can berealized.

FIG. 32D illustrates an example of a portable computer 9800. Thecomputer 9800 has a housing 9801 and a housing 9804 which are joined toeach other so that the portable computer can be opened and closed. Adisplay portion 9802 is incorporated in the housing 9804, and thehousing 9801 is provided with a keyboard 9803 and the like. Note thatthe structure of the computer 9800 is not particularly limited to acertain structure, and the structure may include at least the thin filmtransistor described in Embodiment 1 or 2, and an additional accessorycan be provided as appropriate. A driver circuit and a pixel portion areformed over the same substrate, which leads to reduction inmanufacturing cost. Thus, a computer having a thin film transistor withfavorable electrical characteristics can be realized.

FIG. 33A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display images. Here, the housing9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with operation keys 9609 of the remote controller9610, so that images displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 includes a receiver, a modem, and thelike. With the receiver, general television broadcasting can bereceived. Further, when the television set is connected to wire orwireless communication network through the modem, one-way (from atransmitter to a receiver) or two-way (between a transmitter and areceiver or between receivers) data communication can be performed.

FIG. 33B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displaydata of an image photographed with a digital camera or the like and canfunction in a manner similar to that of a normal photo frame.

Note that the digital photo frame 9700 includes an operation portion, anexternal connection terminal (e.g., a USB terminal or a terminal whichcan be connected to a variety of cables such as USB cables), a recordingmedium insertion portion, and the like. Although these components may beprovided on a surface on which the display portion is provided, it ispreferable to provide them on a side surface or a back surface becausethe design of the digital photo frame is improved. For example, a memorywhich stores data of an image photographed with a digital camera isinserted in the recording medium insertion portion of the digital photoframe, so that the image data can be transferred and displayed on thedisplay portion 9703.

Alternatively, the digital photo frame 9700 may transmit and receivedata wirelessly. Through wireless communication, desired image data canbe transferred and displayed.

FIG. 34A illustrates an example of a mobile phone 1000 which isdifferent from the mobile phone illustrated in FIG. 32C. The mobilephone 1000 includes a display portion 1002 incorporated in a housing1001, an operation button 1003, an external connection port 1004, aspeaker 1005, a microphone 1006, and the like.

In the mobile phone 1000 illustrated in FIG. 34A, data can be input whena person touches the display portion 1002 with his/her finger or thelike. In addition, operations such as making calls and composing mailscan be performed when a person touches the display portion 1002 withhis/her finger or the like.

The display portion 1002 has mainly three screen modes. The first modeis a display mode mainly for displaying images. The second mode is aninput mode mainly for inputting data such as text. The third mode is adisplay-and-input mode in which two modes of the display mode and theinput mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on substantiallyall the area of the screen of the display portion 1002.

By providing a detection device including a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, inside themobile phone 1000, display on the screen of the display portion 1002 canbe automatically changed by determining the orientation of the mobilephone 1000 (whether the mobile phone 1000 is placed horizontally orvertically).

Further, the screen modes are changed by touching the display portion1002 or operating the operation button 1003 of the housing 1001.Alternatively, the screen modes may be changed depending on the kind ofan image displayed on the display portion 1002. For example, when asignal of an image displayed on the display portion is a signal ofmoving image data, the screen mode is changed into the display mode.When the signal is a signal of text data, the screen mode is changedinto the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be changed from the input mode into thedisplay mode.

The display portion 1002 can function also as an image sensor. Forexample, the image of a palm print, a fingerprint, or the like is takenwhen the display portion 1002 is touched with the palm or the finger, sothat authentication can be performed. Further, by using a backlightwhich emits near-infrared light or a sensing light source which emitsnear-infrared light in the display portion, the image of a finger vein,a palm vein, or the like can be taken.

FIG. 34B illustrates an example of a mobile phone. The mobile phone inFIG. 34B includes a display device 9410 in a housing 9411, which has adisplay portion 9412 and operation buttons 9413, and a communicationdevice 9400 in a housing 9401, which has operation buttons 9402, anexternal input terminal 9403, a microphone 9404, a speaker 9405, and alight-emitting portion 9406 which emits light when a phone call isreceived. The display device 9410 which has a display function can bedetached from or attached to the communication device 9400 which has aphone function, in two directions represented by arrows. Thus, thedisplay device 9410 and the communication device 9400 can be attached toeach other along their short sides or long sides. Alternatively, in thecase where only the display function is needed, the display device 9410can be detached from the communication device 9400 and used alone.Images or input data can be transmitted and received by wireless or wirecommunication between the communication device 9400 and the displaydevice 9410 each having a rechargeable battery.

Embodiment 8

In this embodiment, structures and operation of a pixel which can beused in a liquid crystal display device are described. Note that as theoperation mode of a liquid crystal element in this embodiment, a TN(twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringefield switching) mode, an MVA (multi-domain vertical alignment) mode, aPVA (patterned vertical alignment) mode, an ASM (axially symmetricaligned microcell) mode, an OCB (optically compensated birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, or the like can be used.

FIG. 37A illustrates an example of a pixel structure which can be usedin the liquid crystal display device. A pixel 5080 includes a transistor5081, a liquid crystal element 5082, and a capacitor 5083. A gate of thetransistor 5081 is electrically connected to a wiring 5085. A firstterminal of the transistor 5081 is electrically connected to a wiring5084. A second terminal of the transistor 5081 is electrically connectedto a first terminal of the liquid crystal element 5082. A secondterminal of the liquid crystal element 5082 is electrically connected toa wiring 5087. A first terminal of the capacitor 5083 is electricallyconnected to the first terminal of the liquid crystal element 5082. Asecond terminal of the capacitor 5083 is electrically connected to awiring 5086. Note that a first terminal of a transistor is one of asource and a drain, and a second terminal of the transistor is the otherof the source and the drain. That is, when the first terminal of thetransistor is the source, the second terminal of the transistor is thedrain. In a similar manner, when the first terminal of the transistor isthe drain, the second terminal of the transistor is the source.

The wiring 5084 can serve as a signal line. The signal line is a wiringfor transmitting signal voltage, which is input from the outside of thepixel, to the pixel 5080. The wiring 5085 can serve as a scan line. Thescan line is a wiring for controlling on/off of the transistor 5081. Thewiring 5086 can serve as a capacitor line. The capacitor line is awiring for applying predetermined voltage to the second terminal of thecapacitor 5083. The transistor 5081 can serve as a switch. The capacitor5083 can serve as a storage capacitor. The storage capacitor is acapacitor with which the signal voltage is continuously applied to theliquid crystal element 5082 even when the switch is off. The wiring 5087can serve as a counter electrode. The counter electrode is a wiring forapplying predetermined voltage to the second terminal of the liquidcrystal element 5082. Note that the function of each wiring is notlimited to this, and each wiring can have a variety of functions. Forexample, by changing voltage applied to the capacitor line, voltageapplied to the liquid crystal element can be adjusted. Note that it isacceptable as long as the transistor 5081 serves as a switch, and thetransistor 5081 may be either a p-channel transistor or an n-channeltransistor.

FIG. 37B illustrates an example of a pixel structure which can be usedin the liquid crystal display device. The example of the pixel structureillustrated in FIG. 37B is the same as that in FIG. 37A except that thewiring 5087 is eliminated and the second terminal of the liquid crystalelement 5082 and the second terminal of the capacitor 5083 areelectrically connected to each other. The example of the pixel structureillustrated in FIG. 37B can be particularly used in the case of using ahorizontal electric field mode (including an IPS mode and an FFS mode)liquid crystal element. This is because in the horizontal electric fieldmode liquid crystal element, the second terminal of the liquid crystalelement 5082 and the second terminal of the capacitor 5083 can be formedover the same substrate, so that it is easy to electrically connect thesecond terminal of the liquid crystal element 5082 and the secondterminal of the capacitor 5083 to each other. With the pixel structureas illustrated in FIG. 37B, the wiring 5087 can be eliminated, so that amanufacturing process can be simplified and manufacturing cost can bereduced.

A plurality of pixel structures illustrated in FIG. 37A or FIG. 37B canbe arranged in matrix. Thus, a display portion of a liquid crystaldisplay device is formed, and a variety of images can be displayed. FIG.37C illustrates a circuit structure in the case where a plurality ofpixel structures illustrated in FIG. 37A are arranged in matrix. FIG.37C is a circuit diagram illustrating four pixels among a plurality ofpixels included in the display portion. A pixel arranged in an i-thcolumn and a j-th row (each of i and j is a natural number) isrepresented as a pixel 5080_i, j, and a wiring 5084_i, a wiring 5085_j,and a wiring 5086_j are electrically connected to the pixel 5080_i, j.In a similar manner, a wiring 5084_i+1, the wiring 5085_j, and thewiring 5086_j are electrically connected to a pixel 5080_i+1, j. In asimilar manner, the wiring 5084_i, a wiring 5085_j+1, and a wiring5086_j+1 are electrically connected to a pixel 5080_i, j+1. In a similarmanner, the wiring 5084_i+1, the wiring 5085_j+1, and the wiring5086_j+1 are electrically connected to a pixel 5080_i+1, j+1. Note thateach wiring can be used in common with a plurality of pixels in the samerow or the same column. In the pixel structure illustrated in FIG. 37C,the wiring 5087 is a counter electrode, which is used by all the pixelsin common; therefore, the wiring 5087 is not indicated by the naturalnumber i or j. Note that since the pixel structure in FIG. 37B can alsobe used, the wiring 5087 is not required even in a structure where thewiring 5087 is provided and can be eliminated when another wiring servesas the wiring 5087, for example.

The pixel structure in FIG. 37C can be driven by a variety of methods.In particular, when the pixels are driven by a method called AC drive,deterioration (burn-in) of the liquid crystal element can be suppressed.FIG. 37D is a timing chart of voltage applied to each wiring in thepixel structure in FIG. 37C in the case where dot inversion driving,which is a kind of AC drive, is performed. By the dot inversion driving,flickers seen when the AC drive is performed can be suppressed.

In the pixel structure in FIG. 37C, a switch in a pixel electricallyconnected to the wiring 5085_j is selected (in an on state) in a j-thgate selection period in one frame period and is not selected (in an offstate) in the other periods. Then, a (j+1)th gate selection period isprovided after the j-th gate selection period. By performing sequentialscanning in this manner, all the pixels are sequentially selected in oneframe period. In the timing chart of FIG. 37D, the switch in the pixelis selected when the level of voltage is high (high level), and theswitch is not selected when the level of the voltage is low (low level).Note that this is the case where the transistor in each pixel is ann-channel transistor. In the case of using a p-channel transistor, arelationship between voltage and a selection state is opposite to thatin the case of using an n-channel transistor.

In the timing chart illustrated in FIG. 37D, in the j-th gate selectionperiod in a k-th frame (k is a natural number), positive signal voltageis applied to the wiring 5084_i used as a signal line, and negativesignal voltage is applied to the wiring 5084_i+1. Then, in the (j+1)thgate selection period in the k-th frame, negative signal voltage isapplied to the wiring 5084_i, and positive signal voltage is applied tothe wiring 5084_i+1. After that, signals whose polarities are invertedevery gate selection period are alternately supplied to the signal line.Accordingly, in the k-th frame, the positive signal voltage is appliedto the pixels 5080_i, j and 5080_i+1, j+1, and the negative signalvoltage is applied to the pixels 5080_i+1, j and 5080_i,j+1. Then, in a(k+1)th frame, signal voltage whose polarity is opposite to that of thesignal voltage written in the k-th frame is written to each pixel.Accordingly, in the (k+1)th frame, the positive signal voltage isapplied to the pixels 5080_i+1, j and 5080_i, j+1, and the negativesignal voltage is applied to the pixels 5080_i, j and 5080_i+1, j+1. Inthis manner, the dot inversion driving is a driving method by whichsignal voltage whose polarity is different between adjacent pixels isapplied in the same frame and the polarity of the voltage signal for thepixel is inverted every one frame. By the dot inversion driving,flickers seen when the entire or part of an image to be displayed isuniform can be suppressed while deterioration of the liquid crystalelement is suppressed. Note that voltage applied to all the wirings 5086including the wirings 5086_j and 5086_j+1 can be fixed voltage. Notethat although only the polarity of the signal voltage for the wirings5084 is illustrated in the timing chart, the signal voltage can actuallyhave a variety of levels in the polarity illustrated. Note that here,the case where the polarity is inverted per dot (per pixel) isdescribed; however, this embodiment is not limited to this, and thepolarity can be inverted per a plurality of pixels. For example, thepolarity of signal voltage to be written is inverted per two gateselection periods, so that power consumed in writing signal voltage canbe reduced. Alternatively, the polarity can be inverted per column(source line inversion) or per row (gate line inversion).

Note that fixed voltage may be applied to the second terminal of thecapacitor 5083 in the pixel 5080 in one frame period. Here, since thelevel of voltage applied to the wiring 5085 used as a scan line is lowlevel in most of one frame period, which means that substantiallyconstant voltage is applied to the wiring 5085; therefore, the secondterminal of the capacitor 5083 in the pixel 5080 may be connected to thewiring 5085. FIG. 37E illustrates an example of a pixel structure whichcan be used in the liquid crystal display device. Compared to the pixelstructure in FIG. 37C, a feature of the pixel structure in FIG. 37E liesin that the wiring 5086 is eliminated and the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous roware electrically connected to each other. Specifically, in the rangeillustrated in FIG. 37E, the second terminals of the capacitors 5083 inthe pixels 5080_i, j+1 and 5080_i+1, j+1 are electrically connected tothe wiring 5085_j. By electrically connecting the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous rowto each other in this manner, the wiring 5086 can be eliminated, so thatthe aperture ratio of the pixel can be increased. Note that the secondterminal of the capacitor 5083 may be connected to the wiring 5085 inanother row instead of in the previous row. Note that the pixelstructure in FIG. 37E can be driven by a driving method which is similarto that in the pixel structure in FIG. 37C.

Note that voltage applied to the wiring 5084 used as a signal line canbe lowered by using the capacitor 5083 and the wiring electricallyconnected to the second terminal of the capacitor 5083. A pixelstructure and a driving method in this case are described with referenceto FIGS. 37F and 37G Compared to the pixel structure in FIG. 37A, afeature of the pixel structure in FIG. 37F lies in that two wirings 5086are provided per pixel column, and in adjacent pixels, one wiring iselectrically connected to every other second terminal of the capacitors5083 and the other wiring is electrically connected to the remainingevery other second terminal of the capacitors 5083. Note that twowirings 5086 are referred to as a wiring 5086-1 and a wiring 5086-2.Specifically, in the range illustrated in FIG. 37F, the second terminalof the capacitor 5083 in the pixel 5080_i, j is electrically connectedto a wiring 5086-1_j; the second terminal of the capacitor 5083 in thepixel 5080_i+1, j is electrically connected to a wiring 5086-2_j; thesecond terminal of the capacitor 5083 in the pixel 5080_i, j+1 iselectrically connected to a wiring 5086-2_j+1; and the second terminalof the capacitor 5083 in the pixel 5080_i+1, j+1 is electricallyconnected to a wiring 5086-1_j+1.

For example, when positive signal voltage is written to the pixel5080_i, j in the k-th frame as illustrated in FIG. 37G, the wiring5086-1_j becomes a low level, and is changed to a high level after thej-th gate selection period. Then, the wiring 5086-1_j is kept at a highlevel in one frame period, and after negative signal voltage is writtenin the j-th gate selection period in the (k+1)th frame, the wiring5086-1_j is changed to a high level. In this manner, voltage of thewiring which is electrically connected to the second terminal of thecapacitor 5083 is changed in a positive direction after positive signalvoltage is written to the pixel, so that voltage applied to the liquidcrystal element can be changed in the positive direction by apredetermined level. That is, signal voltage written to the pixel can belowered by the predetermined level, so that power consumed in signalwriting can be reduced. Note that when negative signal voltage iswritten in the j-th gate selection period, voltage of the wiring whichis electrically connected to the second terminal of the capacitor 5083is changed in a negative direction after negative signal voltage iswritten to the pixel. Thus, voltage applied to the liquid crystalelement can be changed in the negative direction by a predeterminedlevel, and the signal voltage written to the pixel can be reduced as inthe case of the positive polarity. In other words, as for the wiringwhich is electrically connected to the second terminal of the capacitor5083, different wirings are preferably used for a pixel to whichpositive signal voltage is applied and a pixel to which negative signalvoltage is applied in the same row of the same frame. FIG. 37Fillustrates an example in which the wiring 5086-1 is electricallyconnected to the pixel to which positive signal voltage is applied inthe k-th frame and the wiring 5086-2 is electrically connected to thepixel to which negative signal voltage is applied in the k-th frame.Note that this is just an example, and for example, in the case of usinga driving method by which pixels to which positive signal voltage iswritten and pixels to which negative signal voltage is written appearevery two pixels, it is preferable to perform electrical connectionswith the wirings 5086-1 and 5086-2 alternately every two pixels.Further, in the case where signal voltage of the same polarity iswritten to all the pixels in one row (gate line inversion), one wiring5086 may be provided per row. In other words, in the pixel structure inFIG. 37C, the driving method by which signal voltage written to a pixelis lowered as described with reference to FIGS. 37F and 37G can be used.

Next, a pixel structure and a driving method which are preferably usedparticularly in the case where the mode of a liquid crystal element is avertical alignment (VA) mode typified by an MVA mode and a PVA mode. TheVA mode has advantages such as no rubbing step in manufacture, littlelight leakage at the time of black display, and low driving voltage, buthas a problem in that image quality is decreased (the viewing angle isnarrower) when a screen is seen from an oblique angle. In order to widenthe viewing angle in the VA mode, a pixel structure where one pixelincludes a plurality of subpixels as illustrated in FIGS. 38A and 38B iseffective. Pixel structures illustrated in FIGS. 38A and 38B areexamples of the case where the pixel 5080 includes two subpixels (asubpixel 5080-1 and a subpixel 5080-2). Note that the number ofsubpixels in one pixel is not limited to two and can be other numbers.The viewing angle can be further widened as the number of subpixelsbecomes larger. A plurality of subpixels can have the same circuitstructure. Here, all the subpixels have the circuit structureillustrated in FIG. 37A. Note that the first subpixel 5080-1 includes atransistor 5081-1, a liquid crystal element 5082-1, and a capacitor5083-1. The connection relation of each element is the same as that inthe circuit structure in FIG. 37A. In a similar manner, the secondsubpixel 5080-2 includes a transistor 5081-2, a liquid crystal element5082-2, and a capacitor 5083-2. The connection relation of each elementis the same as that in the circuit structure in FIG. 37A.

The pixel structure in FIG. 38A includes, for two subpixels included inone pixel, two wirings 5085 (a wiring 5085-1 and a wiring 5085-2) usedas scan lines, one wiring 5084 used as a signal line, and one wiring5086 used as a capacitor line. When the signal line and the capacitorline are shared between two subpixels in this manner, the aperture ratiocan be improved. Further, since a signal line driver circuit can besimplified, manufacturing cost can be reduced. Furthermore, since thenumber of connections between a liquid crystal panel and a drivercircuit IC can be reduced, yield can be improved. The pixel structure inFIG. 38B includes, for two subpixels included in one pixel, one wiring5085 used as a scan line, two wirings 5084 (a wiring 5084-1 and a wiring5084-2) used as signal lines, and one wiring 5086 used as a capacitorline. When the scan line and the capacitor line are shared between twosubpixels in this manner, the aperture ratio can be improved. Further,since the total number of scan lines can be reduced, the length of eachgate line selection period can be sufficiently increased even in ahigh-definition liquid crystal panel, and appropriate signal voltage canbe written to each pixel.

FIGS. 38C and 38D illustrate an example in which the liquid crystalelement in the pixel structure in FIG. 38B is replaced with the shape ofa pixel electrode and the electrical connection of each element isschematically illustrated. In FIGS. 38C and 38D, reference numeral5088-1 denotes a first pixel electrode, and reference numeral 5088-2denotes a second pixel electrode. In FIG. 38C, the first pixel electrode5088-1 corresponds to a first terminal of the liquid crystal element5082-1 in FIG. 38B, and the second pixel electrode 5088-2 corresponds toa first terminal of the liquid crystal element 5082-2 in FIG. 38B. Thatis, the first pixel electrode 5088-1 is electrically connected to one ofa source and a drain of the transistor 5081-1, and the second pixelelectrode 5088-2 is electrically connected to one of a source and adrain of the transistor 5081-2. Meanwhile, in FIG. 38D, the connectionrelation between the pixel electrode and the transistor is opposite tothat in FIG. 38C. That is, the first pixel electrode 5088-1 iselectrically connected to one of the source and the drain of thetransistor 5081-2, and the second pixel electrode 5088-2 is electricallyconnected to one of the source and the drain of the transistor 5081-1.

By alternately arranging a plurality of pixel structures as illustratedin FIG. 38C and FIG. 38D in matrix, special advantageous effects can beobtained. FIGS. 39A and 39B illustrate examples of the pixel structureand a driving method thereof. In the pixel structure in FIG. 39A, aportion corresponding to the pixels 5080_i, j and 5080_i+1, j+1 has thestructure illustrated in FIG. 38C, and a portion corresponding to thepixels 5080_i+1, j and 5080_i, j+1 has the structure illustrated in FIG.38D. In this structure, by performing driving as the timing chartillustrated in FIG. 39B, in the j-th gate selection period in the k-thframe, positive signal voltage is written to the first pixel electrodein the pixel 5080_i, j and the second pixel electrode in the pixel5080_i+1, j, and negative signal voltage is written to the second pixelelectrode in the pixel 5080_i, j and the first pixel electrode in thepixel 5080_i+1, j. In the (j+1)th gate selection period in the k-thframe, positive signal voltage is written to the second pixel electrodein the pixel 5080_i, j+1 and the first pixel electrode in the pixel5080_i+1, j+1, and negative signal voltage is written to the first pixelelectrode in the pixel 5080_i, j+1 and the second pixel electrode in thepixel 5080_i+1, j+1. In the (k+1)th frame, the polarity of signalvoltage is inverted in each pixel. Thus, the polarity of voltage appliedto the signal line can be the same in one frame period while drivingcorresponding to dot inversion driving is realized in the pixelstructure including subpixels. Therefore, power consumed in writingsignal voltage to the pixels can be drastically reduced. Note thatvoltage applied to all the wirings 5086 including the wirings 5086_j and5086_j+1 can be fixed voltage.

Further, by a pixel structure and a driving method illustrated in FIGS.39C and 39D, the level of signal voltage written to a pixel can belowered. In the structure, capacitors lines which are electricallyconnected to a plurality of subpixels included in each pixel aredifferent between the subpixels. That is, by using the pixel structureand the driving method illustrated in FIGS. 39C and 39D, subpixels towhich voltages having the same polarities are written in the same frameshare a capacitor line in the same row, and subpixels to which voltageshaving different polarities are written in the same frame use differentcapacitor lines in the same row. Then, when writing in each row isterminated, voltage of the capacitor lines is changed to the positivedirection in the subpixels to which positive signal voltage is written,and changed to the negative direction in the subpixels to which negativesignal voltage is written. Thus, the level of the signal voltage writtento the pixel can be lowered. Specifically, two wirings 5086 (the wirings5086-1 and 5086-2) used as capacitor lines are provided in each row. Thefirst pixel electrode in the pixel 5080_i, j and the wiring 5086-1_j areelectrically connected to each other through the capacitor. The secondpixel electrode in the pixel 5080_i, j and the wiring 5086-2_j areelectrically connected to each other through the capacitor. The firstpixel electrode in the pixel 5080_i+1, j and the wiring 5086-2_j areelectrically connected to each other through the capacitor. The secondpixel electrode in the pixel 5080_i+1, j and the wiring 5086-1_j areelectrically connected to each other through the capacitor. The firstpixel electrode in the pixel 5080_i,j+1 and the wiring 5086-2_j+1 areelectrically connected to each other through the capacitor. The secondpixel electrode in the pixel 5080_i, j+1 and the wiring 5086-1_j+1 areelectrically connected to each other through the capacitor. The firstpixel electrode in the pixel 5080_i+1, j+1 and the wiring 5086-1_j+1 areelectrically connected to each other through the capacitor. The secondpixel electrode in the pixel 5080_i+1, j+1 and the wiring 5086-2_j+1 areelectrically connected to each other through the capacitor. Note thatthis is just an example, and for example, in the case of using a drivingmethod by which pixels to which positive signal voltage is written andpixels to which negative signal voltage is written appear every twopixels, it is preferable to perform electrical connections with thewirings 5086-1 and 5086-2 alternately every two pixels. Further, in thecase where signal voltage of the same polarity is written in all thepixels in one row (gate line inversion), one wiring 5086 may be providedper row. In other words, in the pixel structure in FIG. 39A, the drivingmethod by which signal voltage written to a pixel is lowered asdescribed with reference to FIGS. 39C and 39D can be used.

Embodiment 9

Next, another structure example and a driving method of a display deviceare described. In this embodiment, the case of using a display deviceincluding a display element whose luminance response with respect tosignal writing is slow (response time is long) is described. In thisembodiment, a liquid crystal element is described as an example of thedisplay element with long response time. In this embodiment, a liquidcrystal element is illustrated as an example of the display element withlong response time. However, a display element in this embodiment is notlimited to this, and a variety of display elements whose luminanceresponse with respect to signal writing is slow can be used.

In a general liquid crystal display device, luminance response withrespect to signal writing is slow, and it sometimes takes more than oneframe period to complete the response even when signal voltage iscontinuously applied to a liquid crystal element. Moving images cannotbe displayed precisely by such a display element. Further, in the caseof active matrix driving, time for signal writing to one liquid crystalelement is only a period (one scan line selection period) obtained bydividing a signal writing cycle (one frame period or one subframeperiod) by the number of scan lines, and the liquid crystal elementcannot respond in such a short time in many cases. Therefore, most ofthe response of the liquid crystal element is performed in a periodduring which signal writing is not performed. Here, the dielectricconstant of the liquid crystal element is changed in accordance with thetransmittance of the liquid crystal element, and the response of theliquid crystal element in a period during which signal writing is notperformed means that the dielectric constant of the liquid crystalelement is changed in a state where electric charge is not exchangedwith the outside of the liquid crystal element (in a constant chargestate). In other words, in a formula wherecharge=(capacitance)·(voltage), the capacitance is changed in a statewhere the charge is constant. Accordingly, voltage applied to the liquidcrystal element is changed from voltage in signal writing, in accordancewith the response of the liquid crystal element. Therefore, in the casewhere the liquid crystal element whose luminance response with respectto signal writing is slow is driven by active matrix driving, voltageapplied to the liquid crystal element cannot theoretically reach thevoltage in signal writing.

In the display device in this embodiment, a signal level in signalwriting is corrected in advance (a correction signal is used) so that adisplay element can reach desired luminance within a signal writingcycle. Thus, the above problem can be solved. Further, since theresponse time of the liquid crystal element becomes shorter as thesignal level becomes higher, the response time of the liquid crystalelement can also be shorter by writing a correction signal. A drivingmethod by which such a correction signal is added is referred to asoverdrive. By overdrive in this embodiment, even when a signal writingcycle is shorter than a cycle for an image signal input to the displaydevice (an input image signal cycle T_(in)), the signal level iscorrected in accordance with the signal writing cycle, so that thedisplay element can reach desired luminance within the signal writingcycle. The case where the signal writing cycle is shorter than the inputimage signal cycle T_(in) is, for example, the case where one originalimage is divided into a plurality of subimages and the plurality ofsubimages are sequentially displayed in one frame period.

Next, an example of correcting a signal level in signal writing in adisplay device driven by active matrix driving is described withreference to FIGS. 40A and 40B. FIG. 40A is a graph schematicallyillustrating a time change in luminance of signal level in signalwriting in one display element, with the time as the horizontal axis andthe signal level in signal writing as the vertical axis. FIG. 40B is agraph schematically illustrating a time change in display level, withthe time as the horizontal axis and the display level as the verticalaxis. Note that when the display element is a liquid crystal element,the signal level in signal writing can be voltage, and the display levelcan be the transmittance of the liquid crystal element. In the followingdescription, the vertical axis in FIG. 40A is regarded as the voltage,and the vertical axis in FIG. 40B is regarded as the transmittance. Notethat in the overdrive in this embodiment, the signal level may be otherthan the voltage (may be a duty ratio or current, for example). Notethat in the overdrive in this embodiment, the display level may be otherthan the transmittance (may be luminance or current, for example).Liquid crystal elements are classified into two modes: a normally blackmode in which black is displayed when voltage is 0 (e.g., a VA mode andan IPS mode), and a normally white mode in which white is displayed whenvoltage is 0 (e.g., a TN mode and an OCB mode). The graph illustrated inFIG. 40B corresponds to both of the modes. The transmittance increasesin the upper part of the graph in the normally black mode, and thetransmittance increases in the lower part of the graph in the normallywhite mode. That is, a liquid crystal mode in this embodiment may beeither a normally black mode or a normally white mode. Note that timingof signal writing is represented on the time axis by dotted lines, and aperiod after signal writing is performed until the next signal writingis performed is referred to as a retention period F_(i). In thisembodiment, i is an integer and an index for representing each retentionperiod. In FIGS. 40A and 40B, i is 0 to 2; however, i can be an integerother than 0 to 2 (only the case where i is 0 to 2 is illustrated). Notethat in the retention period F_(i), transmittance for realizingluminance corresponding to an image signal is denoted by T_(i), andvoltage for providing the transmittance T_(i) in a constant state isdenoted by V_(i). In FIG. 40A, a dashed line 5101 represents a timechange in voltage applied to the liquid crystal element in the casewhere overdrive is not performed, and a solid line 5102 represents atime change in voltage applied to the liquid crystal element in the casewhere the overdrive in this embodiment is performed. In a similarmanner, in FIG. 40B, a dashed line 5103 represents a time change intransmittance of the liquid crystal element in the case where overdriveis not performed, and a solid line 5104 represents a time change intransmittance of the liquid crystal element in the case where theoverdrive in this embodiment is performed. Note that a differencebetween the desired transmittance T_(i) and the actual transmittance atthe end of the retention period F_(i) is referred to as an error α_(i).

It is assumed that, in the graph illustrated in FIG. 40A, both thedashed line 5101 and the solid line 5102 represent the case wheredesired voltage V₀ is applied in a retention period F₀; and in the graphillustrated in FIG. 40B, both the dashed line 5103 and the solid line5104 represent the case where desired transmittance T₀ is obtained. Inthe case where overdriving is not performed, desired voltage V₁ isapplied at the beginning of a retention period F₁ as shown by the dashedline 5101. As has been described above, a period for signal writing ismuch shorter than a retention period, and the liquid crystal element isin a constant charge state in most of the retention period. Accordingly,voltage applied to the liquid crystal element in the retention period F₁is changed along with a change in transmittance and is greatly differentfrom the desired voltage V₁ at the end of the retention period F₁. Inthis case, the dashed line 5103 in the graph of FIG. 40B is greatlydifferent from desired transmittance T₁. Accordingly, accurate displayof an image signal cannot be performed, so that image quality isdecreased. On the other hand, in the case where the overdrive in thisembodiment is performed, voltage V₁′ which is higher than the desiredvoltage V₁ is applied to the liquid crystal element at the beginning ofthe retention period F₁ as shown by the solid line 5102. That is, thevoltage V₁′ which is corrected from the desired voltage V₁ is applied tothe liquid crystal element at the beginning of the retention period F₁so that the voltage applied to the liquid crystal element at the end ofthe retention period F₁ is close to the desired voltage V₁ inanticipation of a gradual change in voltage applied to the liquidcrystal element in the retention period F₁. Thus, the desired voltage V₁can be accurately applied to the liquid crystal element. In this case,as shown by the solid line 5104 in the graph of FIG. 40B, the desiredtransmittance T₁ can be obtained at the end of the retention period F₁.In other words, the response of the liquid crystal element within thesignal writing cycle can be realized, despite the fact that the liquidcrystal element is in a constant charge state in most of the retentionperiod. Then, in a retention period F₂, the case where desired voltageV₂ is lower than V₁ is described. Also in that case, as in the retentionperiod F₁, voltage V₂′ which is corrected from the desired voltage V₂may be applied to the liquid crystal element at the beginning of theretention period F₂ so that the voltage applied to the liquid crystalelement at the end of the retention period F₂ is close to the desiredvoltage V₂ in anticipation of a gradual change in voltage applied to theliquid crystal element in the retention period F₂. Thus, as shown by thesolid line 5104 in the graph of FIG. 40B, desired transmittance T₂ canbe obtained at the end of the retention period F₂. Note that in the casewhere V_(i) is higher than V_(i=1) as in the retention period F₁, thecorrected voltage V_(i)′ is preferably corrected so as to be higher thandesired voltage V_(i). Further, when V_(i) is lower than V_(i=1) as inthe retention period F₂, the corrected voltage V_(i)′ is preferablycorrected so as to be lower than the desired voltage V_(i). Note that aspecific correction value can be derived by measuring responsecharacteristics of the liquid crystal element in advance. As a method ofrealizing overdrive in a device, a method by which a correction formulais formulated and included in a logic circuit, a method by which acorrection value is stored in a memory as a look-up table and is read asnecessary, or the like can be used.

Note that there are several limitations on realization of the overdrivein this embodiment in a device. For example, voltage correction has tobe performed in the range of the rated voltage of a source driver. Thatis, in the case where desired voltage is originally high and idealcorrection voltage exceeds the rated voltage of the source driver, notall the correction can be performed. Problems in such a case aredescribed with reference to FIGS. 40C and 40D. As in FIG. 40A, FIG. 40Cis a graph in which a time change in voltage in one liquid crystalelement is schematically illustrated as a solid line 5105 with the timeas the horizontal axis and the voltage as the vertical axis. As in FIG.40B, FIG. 40D is a graph in which a time change in transmittance of oneliquid crystal element is schematically illustrated as a solid line 5106with the time as the horizontal axis and the transmittance as thevertical axis. Note that since other references are similar to those inFIGS. 40A and 40B, description thereof is omitted. FIGS. 40C and 40Dillustrate a state where sufficient correction cannot be performedbecause the correction voltage V₁′ for realizing the desiredtransmittance T₁ in the retention period F₁ exceeds the rated voltage ofthe source driver; thus V₁′=V₁ has to be given. In this case, thetransmittance at the end of the retention period F₁ is deviated from thedesired transmittance T₁ by the error α₁. Note that the error α₁ isincreased only when the desired voltage is originally high; therefore, adecrease in image quality due to occurrence of the error α₁ is in theallowable range in many cases. However, as the error α₁ is increased, anerror in algorithm for voltage correction is also increased. In otherwords, in the algorithm for voltage correction, when it is assumed thatthe desired transmittance is obtained at the end of the retentionperiod, even though the error α₁ is increased, voltage correction isperformed on the basis that the error α₁ is small. Accordingly, theerror is included in correction in the following retention period F₂;thus, an error α₂ is also increased. Further, in the case where theerror α₂ is increased, the following error α₃ is further increased, forexample, and the error is increased in a chain reaction manner, whichresults in a significant decrease in image quality. In the overdrive inthis embodiment, in order to prevent increase of errors in such a chainreaction manner, when the correction voltage V_(i)′ exceeds the ratedvoltage of the source driver in the retention period F_(i), an errorα_(i) at the end of the retention period F_(i) is estimated, and thecorrection voltage in a retention period F_(i+1) can be adjusted inconsideration of the amount of the error α_(i). Thus, even when theerror α_(i) is increased, the effect of the error α_(i) on the errorα_(i+1) can be minimized, so that increase of errors in a chain reactionmanner can be prevented. An example where the error α₂ is minimized inthe overdrive in this embodiment is described with reference to FIGS.40E and 40F. In a graph of FIG. 40E, a solid line 5107 represents a timechange in voltage in the case where the correction voltage V₂′ in thegraph of FIG. 40C is further adjusted to be correction voltage V₂″. Agraph of FIG. 40F illustrates a time change in transmittance in the casewhere voltage is corrected in accordance with the graph of FIG. 40E. Thesolid line 5106 in the graph of FIG. 40D indicates that excessivecorrection is caused by the correction voltage V₂′. On the other hand,the solid line 5108 in the graph of FIG. 40F indicates that excessivecorrection is suppressed by the correction voltage V₂″ which is adjustedin consideration of the error α₁ and the error α₂ is minimized. Notethat a specific correction value can be derived from measuring responsecharacteristics of the liquid crystal element in advance. As a method ofrealizing overdrive in a device, a method by which a correction formulais formulated and included in a logic circuit, a method by which acorrection value is stored in a memory as a look-up table and read asnecessary, or the like can be used. Further, such a method can be addedseparately from a portion for calculating correction voltage V_(i)′ orcan be included in the portion for calculating correction voltageV_(i)′. Note that the amount of correction of correction voltage V_(i)″which is adjusted in consideration of an error α_(i−1) (a differencewith the desired voltage V_(i)) is preferably smaller than that ofV_(i)′. That is, |V_(i)″−V_(i)|<V_(i)′−V_(i)| is preferable.

Note that the error α_(i) which is caused because ideal correctionvoltage exceeds the rated voltage of the source driver is increased as asignal writing cycle becomes shorter. This is because the response timeof the liquid crystal element needs to be shorter as the signal writingcycle becomes shorter, so that higher correction voltage is necessary.Further, as a result of an increase in correction voltage needed, thecorrection voltage exceeds the rated voltage of the source driver morefrequently, so that the large error α_(i) occurs more frequently.Therefore, it can be said that the overdrive in this embodiment becomesmore effective as the signal writing cycle becomes shorter.Specifically, the overdrive in this embodiment is significantlyeffective in the case of performing the following driving methods: adriving method by which one original image is divided into a pluralityof subimages and the plurality of subimages are sequentially displayedin one frame period, a driving method by which motion of an image isdetected from a plurality of images and an intermediate image of theplurality of images is generated and inserted between the plurality ofimages (so-called motion compensation frame rate doubling), and adriving method in which such driving methods are combined, for example.

Note that the rated voltage of the source driver has the lower limit inaddition to the upper limit described above. An example of the lowerlimit is the case where voltage which is lower than the voltage 0 cannotbe applied. In this case, since ideal correction voltage cannot beapplied as in the case of the upper limit described above, the errorα_(i) is increased. However, also in that case, the error α_(i) at theend of the retention period F_(i) is estimated, and the correctionvoltage in the retention period F_(i+1) can be adjusted in considerationof the amount of the error α_(i) in a manner similar to the abovemethod. Note that in the case where voltage which is lower than thevoltage 0 (negative voltage) can be applied as the rated voltage of thesource driver, the negative voltage may be applied to the liquid crystalelement as correction voltage. Thus, the voltage applied to the liquidcrystal element at the end of retention period F_(i) can be adjusted soas to be close to the desired voltage V_(i) in anticipation of a changein potential due to a constant charge state.

Note that in order to suppress deterioration of the liquid crystalelement, so-called inversion driving by which the polarity of voltageapplied to the liquid crystal element is periodically inverted can beperformed in combination with the overdrive. That is, the overdrive inthis embodiment includes the case where the overdrive is performed atthe same time as the inversion driving. For example, in the case wherethe length of the signal writing cycle is half of that of the inputimage signal cycle T_(in), when the length of a cycle for invertingpolarity is the same or substantially the same as that of the inputimage signal cycle T_(in), two sets of writing of a positive signal andtwo sets of writing of a negative signal are alternately performed. Thelength of the cycle for inverting polarity is made larger than that ofthe signal writing cycle in this manner, so that the frequency of chargeand discharge of a pixel can be reduced. Thus, power consumption can bereduced. Note that when the cycle for inverting polarity is made toolong, a defect in which luminance difference due to the difference ofpolarity is recognized as a flicker occurs in some cases; therefore, itis preferable that the length of the cycle for inverting polarity besubstantially the same as or smaller than that of the input image signalcycle T_(in).

Embodiment 10

Next, another structure example and a driving method of a display deviceare described. In this embodiment, a method is described by which animage for interpolating motion of an image input from the outside of adisplay device (an input image) is generated inside the display devicebased on a plurality of input images and the generated image (thegeneration image) and the input image are sequentially displayed. Notethat when an image for interpolating motion of an input image is ageneration image, motion of moving images can be made smooth, and adecrease in quality of moving images because of afterimages or the likedue to hold driving can be suppressed. Here, moving image interpolationis described below. Ideally, display of moving images is realized bycontrolling the luminance of each pixel in real time; however,individual control of pixels in real time has problems such as theenormous number of control circuits, space for wirings, and the enormousamount of input image data. Thus, it is difficult to realize theindividual control of pixels. Therefore, for display of moving images bya display device, a plurality of still images are sequentially displayedin a certain cycle so that display appears to be moving images. Thecycle (in this embodiment, referred to as an input image signal cycleand denoted by T_(in)) is standardized, and for example, 1/60 second inNTSC and 1/50 second in PAL. Such a cycle does not cause a problem ofmoving image display in a CRT, which is an impulsive display device.However, in a hold-type display device, when moving images conforming tothese standards are displayed without change, a defect in which displayis bluffed because of afterimages or the like due to hold driving (holdblur) occurs. Since hold blur is recognized by discrepancy betweenunconscious motion interpolation due to human eyes tracking andhold-type display, the hold blur can be reduced by making the inputimage signal cycle shorter than that in conventional standards (bymaking the control closer to individual control of pixels in real time).However, it is difficult to reduce the length of the input image signalcycle because the standard needs to be changed and the amount of data isincreased. However, an image for interpolating motion of an input imageis generated inside the display device in response to a standardizedinput image signal, and display is performed while the generation imageinterpolates the input image, so that hold blur can be reduced without achange in the standard or an increase in the amount of data. Operationsuch that an image signal is generated inside the display device inresponse to an input image signal to interpolate motion of the inputimage is referred to as moving image interpolation.

By a method for interpolating moving images in this embodiment, motionblur can be reduced. The method for interpolating moving images in thisembodiment can include an image generation method and an image displaymethod. Further, by using a different image generation method and/or adifferent image display method for motion with a specific pattern,motion blur can be effectively reduced. FIGS. 41A and 41B are schematicdiagrams each illustrating an example of a method for interpolatingmoving images in this embodiment. FIGS. 41A and 41B each illustratetiming of treating each image by using the position of the horizontaldirection, with the time as the horizontal axis. A portion representedas “input” indicates timing at which an input image signal is input.Here, images 5121 and 5122 are focused as two images that are temporallyadjacent. An input image is input at an interval of the cycle T_(in).Note that the length of one cycle T_(in) is referred to as one frame orone frame period in some cases. A portion represented as “generation”indicates timing at which a new image is generated from an input imagesignal. Here, an image 5123 which is a generation image generated basedon the images 5121 and 5122 is focused. A portion represented as“display” indicates timing at which an image is displayed in the displaydevice. Note that images other than the focused images are onlyrepresented by dashed lines, and by treating such images in a mannersimilar to that of the focused images, the example of the method forinterpolating moving images in this embodiment can be realized.

In the example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 41A, a generation image which isgenerated based on two input images that are temporally adjacent isdisplayed in a period after one image is displayed until the other imageis displayed, so that moving image interpolation can be performed. Inthis case, a display cycle of a display image is preferably half of aninput cycle of the input image. Note that the display cycle is notlimited to this and can be a variety of display cycles. For example, inthe case where the length of the display cycle is shorter than half ofthat of the input cycle, moving images can be displayed more smoothly.Alternatively, in the case where the length of the display cycle islonger than half of that of the input cycle, power consumption can bereduced. Note that here, an image is generated based on two input imageswhich are temporally adjacent; however, the number of input imagesserving as a basis is not limited to two and can be other numbers. Forexample, when an image is generated based on three (may be more thanthree) input images which are temporally adjacent, a generation imagewith higher accuracy can be obtained as compared to the case where animage is generated based on two input images. Note that the displaytiming of the image 5121 is the same as the input timing of the image5122, that is, the display timing is one frame later than the inputtiming. However, display timing in the method for interpolating movingimages in this embodiment is not limited to this and can be a variety ofdisplay timings. For example, the display timing can be delayed withrespect to the input timing by more than one frame. Thus, the displaytiming of the image 5123 which is the generation image can be delayed,which allows enough time to generate the image 5123 and leads toreduction in power consumption and manufacturing cost. Note that whenthe display timing is delayed with respect to the input timing for along time, a period for holding an input image becomes longer, and thememory capacity which is necessary for holding the input image isincreased. Therefore, the display timing is preferably delayed withrespect to the input timing by approximately one to two frames.

Here, an example of a specific generation method of the image 5123 whichis generated based on the images 5121 and 5122 is described. It isnecessary to detect motion of an input image in order to interpolatemoving images. In this embodiment, a method called a block matchingmethod can be used in order to detect motion of an input image. Notethat this embodiment is not limited to this, and a variety of methods(e.g., a method for obtaining a difference of image data or a method ofusing Fourier transformation) can be used. In the block matching method,first, image data for one input image (here, image data of the image5121) is stored in a data storage means (e.g., a memory circuit such asa semiconductor memory or a RAM). Then, an image in the next frame(here, the image 5122) is divided into a plurality of regions. Note thatthe divided regions can have the same rectangular shapes as illustratedin FIG. 41A; however, the divided regions are not limited to them andcan have a variety of shapes (e.g., the shape or size varies dependingon images). After that, in each divided region, data is compared to theimage data in the previous frame (here, the image data of the image5121), which is stored in the data storage means, so that a region wherethe image data is similar to each other is searched. The example of FIG.41A illustrates that the image 5121 is searched for a region where datais similar to that of a region 5124 in the image 5122, and a region 5126is found. Note that a search range is preferably limited when the image5121 is searched. In the example of FIG. 41A, a region 5125 which isapproximately four times larger than the region 5124 is set as thesearch range. By making the search range larger than this, detectionaccuracy can be increased even in a moving image with high-speed motion.Note that search in an excessively wide range needs an enormous amountof time, which makes it difficult to realize detection of motion. Thus,the region 5125 has preferably approximately two to six times largerthan the area of the region 5124. After that, a difference of theposition between the searched region 5126 and the region 5124 in theimage 5122 is obtained as a motion vector 5127. The motion vector 5127represents motion of image data in the region 5124 in one frame period.Then, in order to generate an image illustrating the intermediate stateof motion, an image generation vector 5128 obtained by changing the sizeof the motion vector without a change in the direction thereof isgenerated, and image data included in the region 5126 of the image 5121is moved in accordance with the image generation vector 5128, so thatimage data in a region 5129 of the image 5123 is generated. Byperforming a series of processings on the entire region of the image5122, the image 5123 can be generated. Then, by sequentially displayingthe input image 5121, the generated image 5123, and the input image5122, moving images can be interpolated. Note that the position of anobject 5130 in the image is different (i.e., the object is moved)between the images 5121 and 5122. In the generated image 5123, theobject is located at the midpoint between the images 5121 and 5122. Bydisplaying such images, motion of moving images can be made smooth, andblur of moving images due to afterimages or the like can be reduced.

Note that the size of the image generation vector 5128 can be determinedin accordance with the display timing of the image 5123. In the exampleof FIG. 41A, since the display timing of the image 5123 is the midpoint(½) between the display timings of the images 5121 and 5122, the size ofthe image generation vector 5128 is half of that of the motion vector5127. Alternatively, for example, when the display timing is ⅓ betweenthe display timings of the images 5121 and 5122, the size of the imagegeneration vector 5128 can be ⅓, and when the display timing is ⅔between the display timings of the images 5121 and 5122, the size can be⅔.

Note that in the case where a new image is generated by moving aplurality of regions having different motion vectors in this manner, aportion where one region has already been moved to a region that is adestination for another region or a portion to which any region is notmoved is generated in some cases (i.e., overlap or blank occurs in somecases). For such portions, data can be compensated. As a method forcompensating an overlap portion, a method by which overlap data isaveraged; a method by which data is arranged in order of priorityaccording to the direction of motion vectors or the like, andhigh-priority data is used as data in a generation image; or a method bywhich one of color and brightness is arranged in order of priority andthe other thereof is averaged can be used, for example. As a method forcompensating a blank portion, a method by which image data of theportion of the image 5121 or the image 5122 is used as data in ageneration image without modification, a method by which image data ofthe portion of the image 5121 or the image 5122 is averaged, or the likecan be used. Then, the generated image 5123 is displayed in accordancewith the size of the image generation vector 5128, so that motion ofmoving images can be made smooth, and the decrease in quality of movingimages because of afterimages or the like due to hold driving can besuppressed.

In another example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 41B, when a generation image which isgenerated based on two input images which are temporally adjacent isdisplayed in a period after one image is displayed until the other imageis displayed, each display image is divided into a plurality ofsubimages to be displayed. Thus, moving images can be interpolated. Thiscase can have advantages of displaying a dark image at regular intervals(advantages when a display method is made closer to impulsive display)in addition to advantages of a shorter image display cycle. That is,blur of moving images due to afterimages or the like can be furtherreduced as compared to the case where the length of the image displaycycle is just made to half of that of the image input cycle. In theexample of FIG. 41B, “input” and “generation” can be similar to theprocessings in the example of FIG. 41A; therefore, description thereofis omitted. For “display” in the example of FIG. 41B, one input imageand/or one generation image can be divided into a plurality of subimagesto be displayed. Specifically, as illustrated in FIG. 41B, the image5121 is divided into subimages 5121 a and 5121 b and the subimages 5121a and 5121 b are sequentially displayed so as to make human eyesperceive that the image 5121 is displayed; the image 5123 is dividedinto subimages 5123 a and 5123 b and the subimages 5123 a and 5123 b aresequentially displayed so as to make human eyes perceive that the image5123 is displayed; and the image 5122 is divided into subimages 5122 aand 5122 b and the subimages 5122 a and 5122 b are sequentiallydisplayed so as to make human eyes perceive that the image 5122 isdisplayed. That is, the display method can be made closer to impulsivedisplay while the image perceived by human eyes is similar to that inthe example of FIG. 41A, so that blur of moving images due toafterimages or the like can be further reduced. Note that the number ofdivision of subimages is two in FIG. 41B; however, the number ofdivision of subimages is not limited to this and can be other numbers.Note that subimages are displayed at regular intervals (½) in FIG. 41B;however, timing of displaying subimages is not limited to this and canbe a variety of timings. For example, when timing of displaying darksubimages 5121 b, 5122 b, and 5123 b is made earlier (specifically,timing at ¼ to ½), the display method can be made much closer toimpulsive display, so that blur of moving images due to afterimages orthe like can be further reduced. Alternatively, when the timing ofdisplaying dark subimages is delayed (specifically, timing at ½ to ¾),the length of a period for displaying a bright image can be increased,so that display efficiency can be increased and power consumption can bereduced.

Another example of the method for interpolating moving images in thisembodiment is an example in which the shape of an object which is movedin an image is detected and different processings are performeddepending on the shape of the moving object. FIG. 41C illustratesdisplay timing as in the example of FIG. 41B and the case where movingcharacters (also referred to as scrolling texts, subtitles, captions, orthe like) are displayed. Note that since terms “input” and “generation”may be similar to those in FIG. 41B, they are not illustrated in FIG.41C. The amount of blur of moving images by hold driving variesdepending on properties of a moving object in some cases. In particular,blur is recognized remarkably when characters are moved in many cases.This is because eyes track moving characters to read the characters, sothat hold blur easily occur. Further, since characters have clearoutlines in many cases, blur due to hold blur is further emphasized insome cases. That is, determining whether an object which is moved in animage is a character and performing special processing when the objectis the character are effective in reducing hold blur. Specifically, whenedge detection, pattern detection, and/or the like are/is performed onan object which is moved in an image and the object is determined to bea character, motion compensation is performed even on subimagesgenerated by division of one image so that an intermediate state ofmotion is displayed. Thus, motion can be made smooth. In the case wherethe object is determined not to be a character, when subimages aregenerated by division of one image as illustrated in FIG. 41B, thesubimages can be displayed without a change in the position of themoving object. The example of FIG. 41C illustrates the case where aregion 5131 determined to be characters is moved upward, and theposition of the region 5131 is different between the subimages 5121 aand 5121 b. In a similar manner, the position of the region 5131 isdifferent between the subimages 5123 a and 5123 b, and between thesubimages 5122 a and 5122 b. Thus, motion of characters for which holdblur is particularly easily recognized can be made smoother than that bynormal motion compensation frame rate doubling, so that blur of movingimages due to afterimages or the like can be further reduced.

This application is based on Japanese Patent Application serial no.2009-051899 filed with Japan Patent Office on Mar. 5, 2009, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a pixelportion over a substrate; a driver circuit portion over the substrate;the pixel portion comprising: a first semiconductor layer comprising achannel formation region; a first electrode electrically connected tothe first semiconductor layer; a first insulating film over the firstsemiconductor layer and the first electrode; a second electrodeoverlapping with the channel formation region with the first insulatingfilm interposed therebetween; a second insulating film over the secondelectrode; a third electrode over and in contact with the secondinsulating film, the third electrode having a region overlapping withthe channel formation region with the second electrode interposedtherebetween; and a fourth electrode over and in contact with the secondinsulating film, the fourth electrode having the same material as thethird electrode, wherein the second electrode has a region overlappedwith the fourth electrode, wherein the third electrode is in contactwith the first electrode through an opening provided in the firstinsulating film and the second insulating film, wherein the opening doesnot overlap with the first semiconductor layer, wherein each of thesecond electrode and the third electrode has a light-transmittingproperty, and the driver circuit portion comprising: a secondsemiconductor layer comprising a channel formation region; the firstinsulating film over the second semiconductor layer; a fifth electrodeover the first insulating film; and a sixth electrode in contact withthe fifth electrode, wherein the fifth electrode has alight-transmitting property, and wherein the sixth electrode has alight-blocking property.
 2. A semiconductor device comprising: a pixelportion over a substrate; a driver circuit portion over the substrate;the pixel portion comprising: a first semiconductor layer comprising achannel formation region; a first electrode electrically connected tothe first semiconductor layer; a first insulating film over the firstsemiconductor layer and the first electrode; a second electrodeoverlapping with the channel formation region with the first insulatingfilm interposed therebetween; a second insulating film over the secondelectrode; a third electrode over and in contact with the secondinsulating film, the third electrode having a region overlapping withthe entire channel formation region with the second electrode interposedtherebetween; and a fourth electrode over and in contact with the secondinsulating film, the fourth electrode having the same material as thethird electrode, wherein the second electrode has a region overlappedwith the fourth electrode, wherein the third electrode is in contactwith the first electrode through an opening provided in the firstinsulating film and the second insulating film, wherein the opening doesnot overlap with the first semiconductor layer, wherein each of thesecond electrode and the third electrode has a light-transmittingproperty, and the driver circuit portion comprising: a secondsemiconductor layer comprising a channel formation region; the firstinsulating film over the second semiconductor layer; a fifth electrodeover the first insulating film; and a sixth electrode in contact withthe fifth electrode, wherein the fifth electrode has alight-transmitting property, and wherein the sixth electrode has alight-blocking property.